Use openssl for `sha256` instead of `sha2`.

[thomcc]

It's unclear to me why sha2 is used here: https://github.com/mozilla/rust-ece/blob/master/src/crypto_backends/openssl.rs#L20.

[jrconlin]

It's kind of required by the hkdf function., and was probably included as part of the example cargo.

The pedigree for the hkdf and sha2 crates look to be fairly good.

I can investigate using openssl only to reduce the amount of dependencies.

[jrconlin]

Hrm. openssl::sha::Sha256 doesn't implement a fair number of traits the HKDF library wants. (Notably digest::Input and digest::FixedOutput) I could probably create a wrapper class for openssl that includes those traits, but not really sure that's the best solution rather than just use the pure rust libraries, since that would be less hacky.

[thomcc]

Honestly you should probably not be using HKDF either. Stuff from the rust-crypto project hasn't gotten any security auditing IIUC.

But I guess rust-openssl doesn't expose openssl's hkdf functions, dang.