Use go generate to write better loops for decoding arrays,
just as we did for encoding. It doesn't help as much,
relatively speaking, but it's still noticeable.

benchmark                          old ns/op     new ns/op     delta
BenchmarkDecodeComplex128Slice     202348        184529        -8.81%
BenchmarkDecodeFloat64Slice        135800        120979        -10.91%
BenchmarkDecodeInt32Slice          121200        105149        -13.24%
BenchmarkDecodeStringSlice         288129        278214        -3.44%

LGTM=rsc
R=rsc
CC=golang-codereviews
https://golang.org/cl/154420044