encoding/json: reduce unmarshal mallocs for unmapped fields
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%