encoding/json: reduce allocated space in Unmarshal

The decodeState type is a large part of the allocated space during Unmarshal.
The errorContext field is infrequently used, and only on error.
Extract it into a pointer and allocate it separate when necessary.

name                old time/op    new time/op    delta
UnmarshalString-8      115ns ± 5%     114ns ± 3%     ~     (p=0.170 n=15+15)
UnmarshalFloat64-8     113ns ± 2%     106ns ± 1%   -6.42%  (p=0.000 n=15+14)
UnmarshalInt64-8      93.3ns ± 1%    86.9ns ± 4%   -6.89%  (p=0.000 n=14+15)

name                old alloc/op   new alloc/op   delta
UnmarshalString-8       192B ± 0%      160B ± 0%  -16.67%  (p=0.000 n=15+15)
UnmarshalFloat64-8      180B ± 0%      148B ± 0%  -17.78%  (p=0.000 n=15+15)
UnmarshalInt64-8        176B ± 0%      144B ± 0%  -18.18%  (p=0.000 n=15+15)

name                old allocs/op  new allocs/op  delta
UnmarshalString-8       2.00 ± 0%      2.00 ± 0%     ~     (all equal)
UnmarshalFloat64-8      2.00 ± 0%      2.00 ± 0%     ~     (all equal)
UnmarshalInt64-8        1.00 ± 0%      1.00 ± 0%     ~     (all equal)

Change-Id: I53f3f468e6c65f77a12e5138a2626455b197012d
Reviewed-on: https://go-review.googlesource.com/c/go/+/271338


Trust: Josh Bleecher Snyder <josharian@gmail.com>
Trust: Daniel Martí <mvdan@mvdan.cc>
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Daniel Martí <mvdan@mvdan.cc>