encoding/json: allocate less in NewEncoder

encoding/json: allocate less in NewEncoder

The *Encoder is almost always garbage. It doesn't need an
encodeState inside of it (and its bytes.Buffer), since it's
only needed locally inside of Encode.

benchmark                 old ns/op    new ns/op    delta
BenchmarkEncoderEncode         2562         2553   -0.35%

benchmark                 old bytes    new bytes    delta
BenchmarkEncoderEncode          283          102  -63.96%

R=r
CC=gobot, golang-dev
https://golang.org/cl/9365044