encoding/json: make use of encodeStatePool in Marshal

encoding/json: make use of encodeStatePool in Marshal

On my system, this seems to be a significant win, with a major
reduction in allocations and minor speed improvement.

name           old time/op    new time/op    delta
CodeMarshal      9.75ms ± 3%    9.24ms ± 1%   -5.21%  (p=0.001 n=5+10)
CodeMarshal-4    4.98ms ± 1%    4.71ms ± 1%   -5.44%  (p=0.001 n=5+10)
CodeMarshal-8    4.80ms ± 0%    4.77ms ± 1%   -0.70%  (p=0.012 n=5+9)

name           old speed      new speed      delta
CodeMarshal     199MB/s ± 3%   210MB/s ± 1%   +5.46%  (p=0.001 n=5+10)
CodeMarshal-4   390MB/s ± 1%   412MB/s ± 1%   +5.76%  (p=0.001 n=5+10)
CodeMarshal-8   404MB/s ± 0%   407MB/s ± 1%   +0.70%  (p=0.012 n=5+9)

name           old alloc/op   new alloc/op   delta
CodeMarshal      4.59MB ± 0%    1.96MB ± 0%  -57.22%  (p=0.000 n=5+9)
CodeMarshal-4    4.59MB ± 0%    2.00MB ± 0%  -56.39%  (p=0.000 n=5+8)
CodeMarshal-8    4.59MB ± 0%    2.06MB ± 0%  -55.05%  (p=0.001 n=5+9)

name           old allocs/op  new allocs/op  delta
CodeMarshal        16.0 ± 0%       1.0 ± 0%  -93.75%  (p=0.000 n=5+10)
CodeMarshal-4      16.0 ± 0%       1.0 ± 0%  -93.75%  (p=0.000 n=5+10)
CodeMarshal-8      16.0 ± 0%       1.0 ± 0%  -93.75%  (p=0.000 n=5+10)

Change-Id: I9d09850d8227f523f861ae1b4ca248c4a4b16aaf
Reviewed-on: https://go-review.googlesource.com/84897
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>