Use a builder to avoid the copy when converting the []byte to a string.

name                  old time/op    new time/op    delta
ByteByteMap-8            796ns ± 5%     700ns ± 1%  -12.00%  (p=0.000 n=9+8)
Map/identity/ASCII-8     123ns ± 8%     126ns ± 7%     ~     (p=0.194 n=10+10)
Map/identity/Greek-8     198ns ± 2%     204ns ± 5%   +2.99%  (p=0.008 n=9+10)
Map/change/ASCII-8       266ns ±10%     202ns ± 3%  -24.19%  (p=0.000 n=10+10)
Map/change/Greek-8       450ns ± 4%     406ns ± 1%   -9.73%  (p=0.000 n=9+10)
MapNoChanges-8          85.4ns ± 3%    90.2ns ±11%   +5.67%  (p=0.000 n=9+10)

name                  old alloc/op   new alloc/op   delta
ByteByteMap-8             416B ± 0%      208B ± 0%  -50.00%  (p=0.000 n=10+10)
Map/identity/ASCII-8     0.00B          0.00B          ~     (all equal)
Map/identity/Greek-8     0.00B          0.00B          ~     (all equal)
Map/change/ASCII-8        128B ± 0%       64B ± 0%  -50.00%  (p=0.000 n=10+10)
Map/change/Greek-8        160B ± 0%       80B ± 0%  -50.00%  (p=0.000 n=10+10)
MapNoChanges-8           0.00B          0.00B          ~     (all equal)

name                  old allocs/op  new allocs/op  delta
ByteByteMap-8             2.00 ± 0%      1.00 ± 0%  -50.00%  (p=0.000 n=10+10)
Map/identity/ASCII-8      0.00           0.00          ~     (all equal)
Map/identity/Greek-8      0.00           0.00          ~     (all equal)
Map/change/ASCII-8        2.00 ± 0%      1.00 ± 0%  -50.00%  (p=0.000 n=10+10)
Map/change/Greek-8        2.00 ± 0%      1.00 ± 0%  -50.00%  (p=0.000 n=10+10)
MapNoChanges-8            0.00           0.00          ~     (all equal)

Fixes #26304

Change-Id: Ideec9dfc29b0b8107f34fc634247081d0031777d
Reviewed-on: https://go-review.googlesource.com/122875Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>