Support crypto/des

[qmuntal]

This PR adds support for the ciphers defined in crypto/des. Note that DES doesn't support CTR nor GCM modes, that's by design.

This is the new public API:

// SupportsDESCipher returns true if NewDESCipher is supported.
func SupportsDESCipher() bool

// SupportsTripleDESCipher returns true if NewTripleDESCipher is supported.
func SupportsTripleDESCipher() bool

func NewDESCipher(key []byte) (cipher.Block, error)

func NewTripleDESCipher(key []byte) (cipher.Block, error)

The PR contains a lot of changes because I've refactored AES so it can share almost all the code with DES.

Please review the PR commit by commit, it will be easier. The commit with more changes is 8cf7f70, in which I moved most of the generalized AES code into cipher.go. The commit ed65759 is not required, as we know the block size for the ciphers we support, but IMO it is more robust to take them from OpenSSL.

[dagood]

A couple suggestions, but LGTM!

[qmuntal]

@ueno @derekparker Please take a look

[qmuntal]

Weekly remainder: @ueno @derekparker please take a look. Thanks!

[ueno]

Looks good to me.