encoding/base32: eliminate alphabet bounds check

encoding/base32: eliminate alphabet bounds check

name              old time/op   new time/op   delta
EncodeToString-4   35.5µs ± 7%   33.3µs ± 6%  -6.27%   (p=0.008 n=10+9)
DecodeString-4      120µs ± 7%    113µs ± 8%  -5.88%  (p=0.011 n=10+10)

name              old speed     new speed     delta
EncodeToString-4  231MB/s ± 8%  247MB/s ± 5%  +6.55%   (p=0.008 n=10+9)
DecodeString-4    109MB/s ± 7%  116MB/s ± 8%  +6.27%  (p=0.011 n=10+10)

Change-Id: I60bf962464179e35b1711617adbc45a822eaece5
Reviewed-on: https://go-review.googlesource.com/45876
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>