encoding/base64, encoding/base32: make DecodeString faster

encoding/base64, encoding/base32: make DecodeString faster

Previously, an input string was stripped of newline
characters at the beginning of DecodeString and then passed
to Decode. Decode again tried to strip newline characters.
That's waste of time.

benchmark                 old MB/s     new MB/s  speedup
BenchmarkDecodeString        38.37        65.20    1.70x

LGTM=dave, bradfitz
R=golang-codereviews, dave, bradfitz
CC=golang-codereviews
https://golang.org/cl/91770051