strings: speed up Map

strings: speed up Map

name                  old time/op  new time/op  delta
ByteByteMap-4         2.03µs ± 2%  1.03µs ± 2%  -49.24%  (p=0.000 n=10+10)
Map/identity/ASCII-4   246ns ± 0%   158ns ± 0%  -35.90%    (p=0.000 n=9+9)
Map/identity/Greek-4   367ns ± 1%   273ns ± 1%  -25.63%  (p=0.000 n=10+10)
Map/change/ASCII-4     582ns ± 1%   324ns ± 1%  -44.34%  (p=0.000 n=10+10)
Map/change/Greek-4     709ns ± 2%   623ns ± 2%  -12.16%  (p=0.000 n=10+10)
MapNoChanges-4         171ns ± 1%   111ns ± 1%  -35.36%   (p=0.000 n=8+10)

Updates #17859

Change-Id: I55d7d261fdc1ce2dcd0ebe23b0fa20b9889bf54c
Reviewed-on: https://go-review.googlesource.com/33201
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>