Option to use openssl cryptography back-ends?

On #392 (closed) I found that OpenSSL's asm cryptography implementations are sometimes much faster than those available in the current RustCrypto crates. Since AES and SHA1 show up in our profiles, maybe we should have a build-time option to use openssl?

(I'm a little confused about whether this is logically a feature, since it isn't purely additive.)

Related to #87.