* Add CommonCrypto-based crypto utils implementation
Ported and adapted from an old (circa 2014-2015) branch of mine.
DH helpers are based on CCBigNum since CCDH doesn't provide acceptable error
reporting, and SecDH interface is a bit weird and limiting. Given that all
mentioned APIs are private, it doesn't seem to matter which one we're using as
any of them could be changed/removed by Apple at any point.
* Switch Xcode project to CommonCrypto backend
