Reduce number of dependencies of llrt_crypto

[Sytten]

The module has too many dependencies, it really pollutes any project trying to import it since some have very deep web of dependencies. For example we pull in two different bigint implementations (num-bigint-dig and crypto-bigint)
We have a lot of duplicate with ring anyway and we should prefer using it. Curve crypto is still not in ring, but planned (briansmith/ring#1771)