encoding/json: use append for Compact and Indent

encoding/json: use append for Compact and Indent

This is part of the effort to reduce direct reliance on bytes.Buffer
so that we can use a buffer with better pooling characteristics.

Avoid direct use of bytes.Buffer in Compact and Indent and
instead modify the logic to rely only on append.
This avoids reliance on the bytes.Buffer.Truncate method,
which makes switching to a custom buffer implementation easier.

Performance:

	name                old time/op    new time/op    delta
	EncodeMarshaler    25.5ns ± 8%    25.7ns ± 9%   ~     (p=0.724 n=10+10)

	name                old alloc/op   new alloc/op   delta
	EncodeMarshaler     4.00B ± 0%     4.00B ± 0%   ~     (all equal)

	name                old allocs/op  new allocs/op  delta
	EncodeMarshaler      1.00 ± 0%      1.00 ± 0%   ~     (all equal)

Updates #27735

Change-Id: I8cded03fab7651d43b5a238ee721f3472530868e
Reviewed-on: https://go-review.googlesource.com/c/go/+/469555
Run-TryBot: Joseph Tsai <joetsai@digital-static.net>
Reviewed-by: Daniel Martí <mvdan@mvdan.cc>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@google.com>
Auto-Submit: Joseph Tsai <joetsai@digital-static.net>
Reviewed-by: Bryan Mills <bcmills@google.com>