Refactor crypto.[ch] into smaller xof+digest module.

Add two new files (crypto_digest.c, crypto_digest.h) as new module of
crypto.[ch].  This new module includes all functions and dependencies related
to digest and xof operations. Those have been removed from crypto.[ch].

Follows #24658.

Signed-off-by: Fernando Fernandez Mancera <ffmancera@riseup.net>