* Move out code that depends on NSS to crypto_digest_nss.c
* Move out code that depends on OpenSSL to crypto_digest_openssl.c
* Keep the general code that is not specific to any of the above in
crypto_digest.c
Fun fact: these files used to be called log.[ch] until we ran into
conflicts with systems having a log.h file. But now that we always
include "lib/log/log.h", we should be fine.
I am calling the crypto library "crypt_ops", since I want
higher-level crypto things to be separated from lower-level ones.
This library will hold only the low-level ones, once we have it
refactored.