encoding/json: fix and optimize marshal for quoted string

Since Go 1.2 every string can be marshaled to JSON without error even if it
contains invalid UTF-8 byte sequences. Therefore there is no need to use
Marshal again for the only reason of enclosing the string in double quotes.
Not using Marshal here also removes the error check as there has not been a
way for Marshal to fail anyway.

name          old time/op    new time/op    delta
Issue34127-4     360ns ± 3%     200ns ± 3%  -44.56%  (p=0.008 n=5+5)

name          old alloc/op   new alloc/op   delta
Issue34127-4     56.0B ± 0%     40.0B ± 0%  -28.57%  (p=0.008 n=5+5)

name          old allocs/op  new allocs/op  delta
Issue34127-4      3.00 ± 0%      2.00 ± 0%  -33.33%  (p=0.008 n=5+5)

Fixes #34154

Change-Id: Ib60dc11980f9b20d8bef2982de7168943d632263
GitHub-Last-Rev: 9b0ac1d4c5318b6bf9ed7930320f2bd755f9939c
GitHub-Pull-Request: golang/go#34127
Reviewed-on: https://go-review.googlesource.com/c/go/+/193604


Reviewed-by: Daniel Martí <mvdan@mvdan.cc>
Run-TryBot: Daniel Martí <mvdan@mvdan.cc>
TryBot-Result: Gobot Gobot <gobot@golang.org>