Add optional openssl accelerated versions of hash functions, loosely based on

Add optional openssl accelerated versions of hash functions, loosely based on
a patch from Elliott Hughes, who said:

[PATCH] Add support for libcrypto for MD5/SHA.

Orders of magnitude faster (for architectures where OpenSSL/BoringSSL
has optimized assembler).

Also adds sha224sum, sha256sum, sha384sum, and sha512sum for folks
building with libcrypto.

The fallback portable C implementations could easily be refactored
to be API-compatible, but I don't know whether they'd stay here or
move to lib/ so I've left that part alone for now.