We can work out how many bytes can be unquoted trivially in
rescanLiteral, which already iterates over a string's bytes.

Removing the extra loop in unquoteBytes simplifies the function and
speeds it up, especially when decoding simple strings, which are common.

While at it, we can remove unnecessary checks like len(s)<2 and
s[0]=='"'. Add a comment explaining why.

name           old time/op    new time/op    delta
CodeDecoder-8    11.2ms ± 0%    11.1ms ± 1%  -1.63%  (p=0.000 n=9+10)

name           old speed      new speed      delta
CodeDecoder-8   173MB/s ± 0%   175MB/s ± 1%  +1.66%  (p=0.000 n=9+10)

Updates #28923.

Change-Id: I2436a3a7f8148a2f7a6a4cdbd7dec6b32ef5e20c
Reviewed-on: https://go-review.googlesource.com/c/go/+/151157
Run-TryBot: Daniel Martí <mvdan@mvdan.cc>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>