crypto: chacha20-generic - add HChaCha20 library function

Refactor the unkeyed permutation part of chacha20_block() into its own
function, then add hchacha20_block() which is the ChaCha equivalent of
HSalsa20 and is an intermediate step towards XChaCha20 (see
https://cr.yp.to/snuffle/xsalsa-20081128.pdf).  HChaCha20 skips the
final addition of the initial state, and outputs only certain words of
the state.  It should not be used for streaming directly.

Reviewed-by: Ard Biesheuvel <[email protected]>
Acked-by: Martin Willi <[email protected]>
Signed-off-by: Eric Biggers <[email protected]>
Signed-off-by: Herbert Xu <[email protected]>

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