strings: reduce allocation for byteReplacer

strings: reduce allocation for byteReplacer

Though it increases the execution time, the function is already quite
fast for most users, the allocation is much more important.

name                       old time/op    new time/op    delta
ByteReplacerWriteString-8    1.23µs ± 0%    2.16µs ± 1%   +75.31%  (p=0.000 n=10+10)

name                       old alloc/op   new alloc/op   delta
ByteReplacerWriteString-8    2.69kB ± 0%    0.00kB       -100.00%  (p=0.000 n=10+10)

name                       old allocs/op  new allocs/op  delta
ByteReplacerWriteString-8      1.00 ± 0%      0.00       -100.00%  (p=0.000 n=10+10)

Change-Id: I6a36df5fcb8e11ef27e6c7b252aa88e869592f3c
Reviewed-on: https://go-review.googlesource.com/c/go/+/424136
Run-TryBot: Cuong Manh Le <cuong.manhle.vn@gmail.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Joedian Reid <joedian@golang.org>