crypto: skcipher - Add top-level skcipher interface

This patch introduces the crypto skcipher interface which aims
to replace both blkcipher and ablkcipher.

It's very similar to the existing ablkcipher interface.  The
main difference is the removal of the givcrypt interface.  In
order to make the transition easier for blkcipher users, there
is a helper SKCIPHER_REQUEST_ON_STACK which can be used to place
a request on the stack for synchronous transforms.

Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>