Digest proxy example

[shamatar]

Proxy digest to sign a hash, but use another hashing algorithm for deterministic nonce generation.

[shamatar]

For some reason

#[test]
    fn recover_signature_from_message()

is broken (and it was in the original I suppose), but I didn't dig into it.

Also, I didn't check for any test vector for nonce generation compared to libsecp256k1 (C library), but from high level those should immediately match.

[gakonst]

Fixed the test, refactored and cherry-picked on #72. Thanks a lot @shamatar!!