strings: speed up ToUpper()/ToLower() by batch writing data with Builder

strings: speed up ToUpper()/ToLower() by batch writing data with Builder

Updates #52371
Updates CL 423874

name                                                                    old time/op    new time/op    delta
ToUpper/#00-10                                                            2.85ns ± 0%    2.81ns ± 0%   -1.31%  (p=0.000 n=10+10)
ToUpper/ONLYUPPER-10                                                      12.7ns ± 0%    12.5ns ± 0%   -1.35%  (p=0.000 n=10+10)
ToUpper/abc-10                                                            20.9ns ± 1%    20.1ns ± 1%   -3.92%  (p=0.000 n=8+10)
ToUpper/AbC123-10                                                         26.9ns ± 1%    28.5ns ± 0%   +5.78%  (p=0.000 n=9+9)
ToUpper/azAZ09_-10                                                        27.4ns ± 1%    24.5ns ± 0%  -10.82%  (p=0.000 n=9+9)
ToUpper/longStrinGwitHmixofsmaLLandcAps-10                                95.9ns ± 1%   100.3ns ± 0%   +4.52%  (p=0.000 n=9+10)
ToUpper/RENAN_BASTOS_93_AOSDAJDJAIDJAIDAJIaidsjjaidijadsjiadjiOOKKO-10     188ns ± 0%     121ns ± 0%  -35.52%  (p=0.000 n=9+10)
ToUpper/longɐstringɐwithɐnonasciiⱯchars-10                                 168ns ± 0%     164ns ± 0%   -2.02%  (p=0.000 n=8+10)
ToUpper/ɐɐɐɐɐ-10                                                           134ns ± 0%     132ns ± 0%   -1.59%  (p=0.000 n=9+10)
ToUpper/a\u0080\U0010ffff-10                                              67.6ns ± 0%    66.4ns ± 0%   -1.73%  (p=0.000 n=10+10)
ToLower/#00-10                                                            2.87ns ± 4%    2.83ns ± 0%   -1.46%  (p=0.004 n=9+9)
ToLower/abc-10                                                            6.35ns ± 0%    6.29ns ± 0%   -0.98%  (p=0.000 n=9+9)
ToLower/AbC123-10                                                         25.6ns ± 1%    28.1ns ± 1%   +9.81%  (p=0.000 n=10+10)
ToLower/azAZ09_-10                                                        29.9ns ± 1%    30.1ns ± 1%   +0.64%  (p=0.023 n=9+10)
ToLower/longStrinGwitHmixofsmaLLandcAps-10                                96.7ns ± 1%    73.0ns ± 0%  -24.50%  (p=0.000 n=10+10)
ToLower/renan_bastos_93_AOSDAJDJAIDJAIDAJIaidsjjaidijadsjiadjiOOKKO-10     177ns ± 0%     118ns ± 0%  -33.61%  (p=0.000 n=7+8)
ToLower/LONGⱯSTRINGⱯWITHⱯNONASCIIⱯCHARS-10                                 159ns ± 1%     158ns ± 0%   -0.97%  (p=0.000 n=8+10)
ToLower/ⱭⱭⱭⱭⱭ-10                                                           125ns ± 1%     123ns ± 1%   -1.67%  (p=0.000 n=9+9)
ToLower/A\u0080\U0010ffff-10                                              68.4ns ± 1%    67.1ns ± 0%   -1.95%  (p=0.000 n=9+9)

name                                                                    old alloc/op   new alloc/op   delta
ToUpper/#00-10                                                             0.00B          0.00B          ~     (all equal)
ToUpper/ONLYUPPER-10                                                       0.00B          0.00B          ~     (all equal)
ToUpper/abc-10                                                             3.00B ± 0%     3.00B ± 0%     ~     (all equal)
ToUpper/AbC123-10                                                          8.00B ± 0%     8.00B ± 0%     ~     (all equal)
ToUpper/azAZ09_-10                                                         8.00B ± 0%     8.00B ± 0%     ~     (all equal)
ToUpper/longStrinGwitHmixofsmaLLandcAps-10                                 32.0B ± 0%     32.0B ± 0%     ~     (all equal)
ToUpper/RENAN_BASTOS_93_AOSDAJDJAIDJAIDAJIaidsjjaidijadsjiadjiOOKKO-10     64.0B ± 0%     64.0B ± 0%     ~     (all equal)
ToUpper/longɐstringɐwithɐnonasciiⱯchars-10                                 48.0B ± 0%     48.0B ± 0%     ~     (all equal)
ToUpper/ɐɐɐɐɐ-10                                                           48.0B ± 0%     48.0B ± 0%     ~     (all equal)
ToUpper/a\u0080\U0010ffff-10                                               16.0B ± 0%     16.0B ± 0%     ~     (all equal)
ToLower/#00-10                                                             0.00B          0.00B          ~     (all equal)
ToLower/abc-10                                                             0.00B          0.00B          ~     (all equal)
ToLower/AbC123-10                                                          8.00B ± 0%     8.00B ± 0%     ~     (all equal)
ToLower/azAZ09_-10                                                         8.00B ± 0%     8.00B ± 0%     ~     (all equal)
ToLower/longStrinGwitHmixofsmaLLandcAps-10                                 32.0B ± 0%     32.0B ± 0%     ~     (all equal)
ToLower/renan_bastos_93_AOSDAJDJAIDJAIDAJIaidsjjaidijadsjiadjiOOKKO-10     64.0B ± 0%     64.0B ± 0%     ~     (all equal)
ToLower/LONGⱯSTRINGⱯWITHⱯNONASCIIⱯCHARS-10                                 48.0B ± 0%     48.0B ± 0%     ~     (all equal)
ToLower/ⱭⱭⱭⱭⱭ-10                                                           24.0B ± 0%     24.0B ± 0%     ~     (all equal)
ToLower/A\u0080\U0010ffff-10                                               16.0B ± 0%     16.0B ± 0%     ~     (all equal)

name                                                                    old allocs/op  new allocs/op  delta
ToUpper/#00-10                                                              0.00           0.00          ~     (all equal)
ToUpper/ONLYUPPER-10                                                        0.00           0.00          ~     (all equal)
ToUpper/abc-10                                                              1.00 ± 0%      1.00 ± 0%     ~     (all equal)
ToUpper/AbC123-10                                                           1.00 ± 0%      1.00 ± 0%     ~     (all equal)
ToUpper/azAZ09_-10                                                          1.00 ± 0%      1.00 ± 0%     ~     (all equal)
ToUpper/longStrinGwitHmixofsmaLLandcAps-10                                  1.00 ± 0%      1.00 ± 0%     ~     (all equal)
ToUpper/RENAN_BASTOS_93_AOSDAJDJAIDJAIDAJIaidsjjaidijadsjiadjiOOKKO-10      1.00 ± 0%      1.00 ± 0%     ~     (all equal)
ToUpper/longɐstringɐwithɐnonasciiⱯchars-10                                  1.00 ± 0%      1.00 ± 0%     ~     (all equal)
ToUpper/ɐɐɐɐɐ-10                                                            2.00 ± 0%      2.00 ± 0%     ~     (all equal)
ToUpper/a\u0080\U0010ffff-10                                                1.00 ± 0%      1.00 ± 0%     ~     (all equal)
ToLower/#00-10                                                              0.00           0.00          ~     (all equal)
ToLower/abc-10                                                              0.00           0.00          ~     (all equal)
ToLower/AbC123-10                                                           1.00 ± 0%      1.00 ± 0%     ~     (all equal)
ToLower/azAZ09_-10                                                          1.00 ± 0%      1.00 ± 0%     ~     (all equal)
ToLower/longStrinGwitHmixofsmaLLandcAps-10                                  1.00 ± 0%      1.00 ± 0%     ~     (all equal)
ToLower/renan_bastos_93_AOSDAJDJAIDJAIDAJIaidsjjaidijadsjiadjiOOKKO-10      1.00 ± 0%      1.00 ± 0%     ~     (all equal)
ToLower/LONGⱯSTRINGⱯWITHⱯNONASCIIⱯCHARS-10                                  1.00 ± 0%      1.00 ± 0%     ~     (all equal)
ToLower/ⱭⱭⱭⱭⱭ-10                                                            1.00 ± 0%      1.00 ± 0%     ~     (all equal)
ToLower/A\u0080\U0010ffff-10                                                1.00 ± 0%      1.00 ± 0%     ~     (all equal)

Change-Id: Id3998ac4bae054ba3e6cf30545a257d5992b48be
Reviewed-on: https://go-review.googlesource.com/c/go/+/424139
Run-TryBot: Ian Lance Taylor <iant@google.com>
Auto-Submit: Ian Lance Taylor <iant@google.com>
Run-TryBot: Cuong Manh Le <cuong.manhle.vn@gmail.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@google.com>
Reviewed-by: Joedian Reid <joedian@golang.org>