bytes, strings: speed up TrimSpace 4-5x for common ASCII cases

bytes, strings: speed up TrimSpace 4-5x for common ASCII cases

This change adds a fast path for ASCII strings to both
strings.TrimSpace and bytes.TrimSpace. It doesn't slow down the
non-ASCII path much, if at all.

I added benchmarks for strings.TrimSpace as it didn't have any, and
I fleshed out the benchmarks for bytes.TrimSpace as it just had one
case (for ASCII). The benchmarks (and the code!) are now the same
between the two versions. Below are the benchmark results:

strings.TrimSpace:

name                      old time/op  new time/op  delta
TrimSpace/NoTrim-8        18.6ns ± 0%   3.8ns ± 0%  -79.53%  (p=0.000 n=5+4)
TrimSpace/ASCII-8         33.5ns ± 2%   6.0ns ± 3%  -82.05%  (p=0.008 n=5+5)
TrimSpace/SomeNonASCII-8  97.1ns ± 1%  88.6ns ± 1%   -8.68%  (p=0.008 n=5+5)
TrimSpace/JustNonASCII-8   144ns ± 0%   143ns ± 0%     ~     (p=0.079 n=4+5)

bytes.TrimSpace:

name                      old time/op  new time/op  delta
TrimSpace/NoTrim-8        18.9ns ± 1%   4.1ns ± 1%  -78.34%  (p=0.008 n=5+5)
TrimSpace/ASCII-8         29.9ns ± 0%   6.3ns ± 1%  -79.06%  (p=0.008 n=5+5)
TrimSpace/SomeNonASCII-8  91.5ns ± 0%  82.3ns ± 0%  -10.03%  (p=0.008 n=5+5)
TrimSpace/JustNonASCII-8   150ns ± 0%   150ns ± 0%     ~     (all equal)

Fixes #29122

Change-Id: Ica45cd86a219cadf60173ec9db260133cd1d7951
Reviewed-on: https://go-review.googlesource.com/c/go/+/152917
Reviewed-by: Daniel Martí <mvdan@mvdan.cc>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Daniel Martí <mvdan@mvdan.cc>
TryBot-Result: Gobot Gobot <gobot@golang.org>