requires a decoder to do its own byte buffering instead of using
bufio.Reader, due to byte stuffing.

benchmark                      old MB/s     new MB/s     speedup
BenchmarkDecodeBaseline        33.40        50.65        1.52x
BenchmarkDecodeProgressive     24.34        31.92        1.31x

On 6g, unsafe.Sizeof(huffman{}) falls from 4872 to 964 bytes, and
the decoder struct contains 8 of those.

LGTM=r
R=r, nightlyone
CC=bradfitz, couchmoney, golang-codereviews, raph
https://golang.org/cl/109050045