crypto: chacha - move existing library code into lib/crypto

Currently, our generic ChaCha implementation consists of a permute
function in lib/chacha.c that operates on the 64-byte ChaCha state
directly [and which is always included into the core kernel since it
is used by the /dev/random driver], and the crypto API plumbing to
expose it as a skcipher.

In order to support in-kernel users that need the ChaCha streamcipher
but have no need [or tolerance] for going through the abstractions of
the crypto API, let's expose the streamcipher bits via a library API
as well, in a way that permits the implementation to be superseded by
an architecture specific one if provided.

So move the streamcipher code into a separate module in lib/crypto,
and expose the init() and crypt() routines to users of the library.

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