Use openssl instead of ring.

[eoger]

We are using rust-hawk in some applications that have a dependency on rust-openssl for different reasons.
rust-hawk itself depends on ring (2.2mb compiled with cargo build --release) for operations that could be done with rust-openssl instead, hence this PR.

[thomcc]

Echoing what I said in IRC for posterity -- I think we should be more careful about unwraps, and either know for sure they're safe, or expose them as Results to the caller. It is annoying that rust-openssl and ring are not fallible for the same operations, certainly.

I also dunno if we want to maintain a fork we never upstream for this. If we're upstreaming it then we might want to reach out to the maintainer and ask if they'd accept this -- if it were me I would want this implemented as a feature.

If this is too much work then maybe we can put off worrying about it until we get things working, though.

[eoger]

Unwraps removed, thanks!
I think we should put this work under a feature and upstream it once rust-lang/cargo#1839 is resolved:
We want to depend on ring if use-ring is specified, and openssl if use-openssl is specified.
Optional dependencies would make this crate clunky to use imo.