This commit adds the cbcEncAble and cbcDecAble interfaces that
can be implemented by block ciphers that support an optimized
implementation of CBC. This is similar to what is done for GCM
with the gcmAble interface.

The cbcEncAble, cbcDecAble and gcmAble interfaces all now have
tests to ensure they are detected correctly in the cipher
package.

name             old speed     new speed      delta
AESCBCEncrypt1K  152MB/s ± 1%  1362MB/s ± 0%  +795.59%   (p=0.000 n=10+9)
AESCBCDecrypt1K  143MB/s ± 1%  1362MB/s ± 0%  +853.00%   (p=0.000 n=10+9)

Change-Id: I715f686ab3686b189a3dac02f86001178fa60580
Reviewed-on: https://go-review.googlesource.com/22523
Run-TryBot: Michael Munday <munday@ca.ibm.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Adam Langley <agl@golang.org>