decrypt: reduced the number of copy calls from 2n to 1.
encrypt: reduced the number of copy calls from n to 1.

Encryption is straight-forward: use dst instead of tmp when
xoring the block with the iv.

Decryption now loops backwards through the blocks abusing the
fact that the previous block's ciphertext (src) is the iv. This
means we don't need to copy the iv every time, in addition to
using dst instead of tmp like encryption.

R=golang-codereviews, agl, mikioh.mikioh
CC=golang-codereviews
https://golang.org/cl/50900043