JSON decoding performs poorly for unmapped and ignored fields. We noticed better
performance when unmarshalling unused fields. The loss comes mostly from calls
to scanner.error as described at #17914.

benchmark                 old ns/op     new ns/op     delta
BenchmarkIssue10335-8     431           408           -5.34%
BenchmarkUnmapped-8       1744          1314          -24.66%

benchmark                 old allocs     new allocs     delta
BenchmarkIssue10335-8     4              3              -25.00%
BenchmarkUnmapped-8       18             4              -77.78%

benchmark                 old bytes     new bytes     delta
BenchmarkIssue10335-8     320           312           -2.50%
BenchmarkUnmapped-8       568           344           -39.44%

Fixes #17914, improves #10335

Change-Id: I7d4258a94eb287c0fe49e7334795209b90434cd0
Reviewed-on: https://go-review.googlesource.com/33276Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>