Store already padded keys instead of storing key and padding it during
Reset and Sum. This simplifies code and makes Reset-Write-Sum sequences
faster, which helps /x/crypto/pbkdf2.

HMAC benchmark:

benchmark                    old ns/op     new ns/op     delta
BenchmarkHMACSHA256_1K-4     7669          7613          -0.73%
BenchmarkHMACSHA256_32-4     1880          1737          -7.61%

benchmark                    old MB/s     new MB/s     speedup
BenchmarkHMACSHA256_1K-4     133.52       134.50       1.01x
BenchmarkHMACSHA256_32-4     17.02        18.41        1.08x

PBKDF2 benchmark:

benchmark                       old ns/op     new ns/op     delta
BenchmarkPBKDF2HMACSHA256-4     1943196       1807699       -6.97%

Change-Id: I6697028370c226715ab477b0844951a83eb3488c
Reviewed-on: https://go-review.googlesource.com/21024
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Adam Langley <agl@golang.org>