Add support for noncedata in secp.sign_ecdsa and secp.sign_ecdsa_recoverable

[junderw]

I would like to add (and am willing to make a PR) these two new methods.

I propose the names be sign_ecdsa_with_noncedata and sign_ecdsa_recoverable_with_noncedata and their function signatures add an extra argument at the end of type [u8; 32] and called noncedata.

Questions:

1. I would be willing to do this PR, but I want to get an idea of the willingness to add this to the API. Any objections?
2. Should I also add a new wrapper struct (similar to the other structs such as Message etc.)?

[apoelstra]

1. Sure, we'll take the new methods.
2. No, no reason to wrap the data. It's raw, unstructured data and has no security consequence.