[RFC] Use CommonCrypto for hashing

OSX ships with its own crypto library that we link to; prefer it to OpenSSL when we're there. This just activates it for hashing and still goes through OpenSSL for network communication stuff.

Reading between the lines, the documentation seems to be saying that their hashing code is just the OpenSSL one, which is fair enough since the crypto stuff actually decent, but this still represents a step in reducing the dependency on OpenSSL.