encoding/pem: skip whitespace work on most inputs

encoding/base64 already skips \r and \n when decoding, so this package
must only deal with spaces and tabs. Those aren't nearly as common, so
we can add a fast path with bytes.ContainsAny to skip the costly alloc
and filtering code.

name      old time/op    new time/op    delta
Decode-8     279µs ± 0%     259µs ± 1%   -7.07%  (p=0.002 n=6+6)

name      old speed      new speed      delta
Decode-8   319MB/s ± 0%   343MB/s ± 1%   +7.61%  (p=0.002 n=6+6)

name      old alloc/op   new alloc/op   delta
Decode-8     164kB ± 0%      74kB ± 0%  -54.90%  (p=0.002 n=6+6)

name      old allocs/op  new allocs/op  delta
Decode-8      12.0 ± 0%      11.0 ± 0%   -8.33%  (p=0.002 n=6+6)

Change-Id: Idfca8700c52f46eb70a4a7e0d2db3bf0124e4699
Reviewed-on: https://go-review.googlesource.com/c/155964


Run-TryBot: Daniel Martí <mvdan@mvdan.cc>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>