Cryptographic module abstraction

At present, apk-tools makes direct use of libcrypto to do signature verification, signing, etc.

It would be nice to factor this out so that we can eventually drop the libcrypto dependency with something better.

Similarly, it would be nice to replace our use of libssl with libtls in our libfetch fork, for the same reason: multiple libtls implementations exist, and we can just use whatever one we want (for now, libtls-standalone, but later perhaps libtls-bearssl instead.)