encoding/base64: Optimize EncodeToString and DecodeString.

benchmark                   old ns/op     new ns/op     delta
BenchmarkEncodeToString     31281         23821         -23.85%
BenchmarkDecodeString       156508        82254         -47.44%

benchmark                   old MB/s     new MB/s     speedup
BenchmarkEncodeToString     261.88       343.89       1.31x
BenchmarkDecodeString       69.80        132.81       1.90x

Change-Id: I115e0b18c3a6d5ef6bfdcb3f637644f02f290907
Reviewed-on: https://go-review.googlesource.com/8808


Reviewed-by: Nigel Tao <nigeltao@golang.org>