crypto: chacha20poly1305 - import construction and selftest from Zinc

This incorporates the chacha20poly1305 from the Zinc library, retaining
the library interface, but replacing the implementation with calls into
the code that already existed in the kernel's crypto API.

Note that this library API does not implement RFC7539 fully, given that
it is limited to 64-bit nonces. (The 96-bit nonce version that was part
of the selftest only has been removed, along with the 96-bit nonce test
vectors that only tested the selftest but not the actual library itself)

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