runtime: replace stringStruct with unsafe.String where appropriate

runtime: replace stringStruct with unsafe.String where appropriate

Simplify the code a bit, no significant performance changes.

name                             old time/op    new time/op    delta
HashStringSpeed-8                  9.64ns ±11%    8.91ns ± 9%   -7.60%  (p=0.007 n=10+10)
HashStringArraySpeed-8             19.8ns ± 3%    19.5ns ± 2%     ~     (p=0.085 n=10+10)
MapStringKeysEight_16-8            10.7ns ± 3%    10.2ns ± 2%   -4.48%  (p=0.000 n=10+10)
MapStringKeysEight_32-8            8.89ns ± 3%    8.71ns ± 3%     ~     (p=0.082 n=9+10)
MapStringKeysEight_64-8            8.84ns ± 2%    8.60ns ± 3%   -2.73%  (p=0.004 n=9+10)
MapStringKeysEight_1M-8            8.90ns ± 3%    8.62ns ± 3%   -3.15%  (p=0.000 n=10+10)
MapStringConversion/32/simple-8    8.62ns ± 3%    8.61ns ± 2%     ~     (p=0.895 n=10+10)
MapStringConversion/32/struct-8    8.53ns ± 2%    8.63ns ± 2%     ~     (p=0.123 n=10+10)
MapStringConversion/32/array-8     8.54ns ± 2%    8.50ns ± 1%     ~     (p=0.590 n=9+9)
MapStringConversion/64/simple-8    8.44ns ± 2%    8.38ns ± 2%     ~     (p=0.353 n=10+10)
MapStringConversion/64/struct-8    8.41ns ± 2%    8.48ns ± 2%     ~     (p=0.143 n=10+10)
MapStringConversion/64/array-8     8.42ns ± 2%    8.44ns ± 2%     ~     (p=0.739 n=10+10)
MapInterfaceString-8               13.6ns ±26%    13.6ns ±20%     ~     (p=0.736 n=10+9)
AppendGrowString-8                 38.9ms ± 9%    40.2ms ±13%     ~     (p=0.481 n=10+10)
CompareStringEqual-8               3.03ns ± 2%    2.86ns ± 3%   -5.58%  (p=0.000 n=10+10)
CompareStringIdentical-8           1.20ns ± 3%    1.01ns ± 4%  -16.16%  (p=0.000 n=10+10)
CompareStringSameLength-8          2.11ns ± 3%    1.85ns ± 3%  -12.33%  (p=0.000 n=10+10)
CompareStringDifferentLength-8     0.30ns ± 0%    0.30ns ± 0%     ~     (p=0.508 n=10+9)
CompareStringBigUnaligned-8        43.0µs ± 1%    42.8µs ± 2%     ~     (p=0.165 n=10+10)
CompareStringBig-8                 43.2µs ± 2%    43.4µs ± 2%     ~     (p=0.661 n=9+10)
ConcatStringAndBytes-8             15.1ns ± 1%    14.9ns ± 1%   -1.57%  (p=0.001 n=8+10)
SliceByteToString/1-8              2.45ns ± 2%    2.39ns ± 2%   -2.64%  (p=0.000 n=10+10)
SliceByteToString/2-8              10.9ns ± 2%    10.8ns ± 4%     ~     (p=0.060 n=10+10)
SliceByteToString/4-8              11.9ns ± 0%    11.8ns ± 1%   -0.97%  (p=0.000 n=8+8)
SliceByteToString/8-8              13.9ns ± 1%    13.9ns ± 1%   +0.57%  (p=0.009 n=9+9)
SliceByteToString/16-8             18.0ns ± 3%    18.6ns ± 5%   +2.78%  (p=0.001 n=9+10)
SliceByteToString/32-8             20.1ns ± 3%    20.5ns ± 5%   +2.10%  (p=0.034 n=10+10)
SliceByteToString/64-8             24.3ns ± 3%    24.9ns ± 3%   +2.28%  (p=0.001 n=9+10)
SliceByteToString/128-8            33.8ns ± 1%    34.5ns ± 4%     ~     (p=0.264 n=8+10)

Updates #54854

Change-Id: I7ce57a92c5f590fa8cb31a48969d281147eb05f1
Reviewed-on: https://go-review.googlesource.com/c/go/+/428759
Reviewed-by: hopehook <hopehook@golangcn.org>
Reviewed-by: Keith Randall <khr@google.com>
Run-TryBot: Ian Lance Taylor <iant@google.com>
Auto-Submit: Ian Lance Taylor <iant@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Cuong Manh Le <cuong.manhle.vn@gmail.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>