bytes: optimize ToLower and ToUpper for ASCII-only case

bytes: optimize ToLower and ToUpper for ASCII-only case

Follow what CL 68370 and CL 76470 did for the respective functions in
package strings.

Also adjust godoc strings to match the respective strings functions and
mention the special case for ASCII-only byte slices which don't need
conversion.

name                                       old time/op  new time/op  delta
ToUpper/#00-8                              9.35ns ± 3%  6.08ns ± 2%  -35.04%  (p=0.000 n=9+9)
ToUpper/ONLYUPPER-8                        77.7ns ± 1%  16.9ns ± 2%  -78.22%  (p=0.000 n=10+10)
ToUpper/abc-8                              36.5ns ± 1%  22.1ns ± 1%  -39.43%  (p=0.000 n=10+8)
ToUpper/AbC123-8                           56.9ns ± 2%  28.2ns ± 2%  -50.54%  (p=0.000 n=8+10)
ToUpper/azAZ09_-8                          62.3ns ± 1%  26.9ns ± 1%  -56.82%  (p=0.000 n=9+10)
ToUpper/longStrinGwitHmixofsmaLLandcAps-8   219ns ± 2%    63ns ± 2%  -71.17%  (p=0.000 n=10+10)
ToUpper/longɐstringɐwithɐnonasciiⱯchars-8   367ns ± 2%   374ns ± 3%   +2.05%  (p=0.000 n=9+10)
ToUpper/ɐɐɐɐɐ-8                             200ns ± 1%   206ns ± 1%   +2.49%  (p=0.000 n=10+10)
ToUpper/a\u0080\U0010ffff-8                90.4ns ± 1%  93.8ns ± 0%   +3.82%  (p=0.000 n=10+7)
ToLower/#00-8                              9.59ns ± 1%  6.13ns ± 2%  -36.08%  (p=0.000 n=10+10)
ToLower/abc-8                              36.4ns ± 1%  10.4ns ± 1%  -71.50%  (p=0.000 n=10+10)
ToLower/AbC123-8                           55.8ns ± 1%  27.5ns ± 1%  -50.61%  (p=0.000 n=10+10)
ToLower/azAZ09_-8                          61.7ns ± 1%  30.2ns ± 1%  -50.98%  (p=0.000 n=8+10)
ToLower/longStrinGwitHmixofsmaLLandcAps-8   226ns ± 1%    64ns ± 1%  -71.53%  (p=0.000 n=10+9)
ToLower/LONGⱯSTRINGⱯWITHⱯNONASCIIⱯCHARS-8   354ns ± 0%   361ns ± 0%   +2.18%  (p=0.000 n=10+10)
ToLower/ⱭⱭⱭⱭⱭ-8                             180ns ± 1%   186ns ± 0%   +3.45%  (p=0.000 n=10+9)
ToLower/A\u0080\U0010ffff-8                91.7ns ± 0%  94.5ns ± 0%   +2.99%  (p=0.000 n=10+10)

Change-Id: Ifdb8ae328ff9feacd1c170db8eebbf98c399e204
Reviewed-on: https://go-review.googlesource.com/c/go/+/170954
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>