2df51dae15 Create crypto module (Tobin C. Harding)
Pull request description:
Done as part of [util flattening](https://github.com/rust-bitcoin/rust-bitcoin/issues/639).
Create a `crypto` module and move into it (out of `util`):
- ecdsa
- schnorr
- key
After review, this PR now includes some type re-names
- EcdsaSig -> ecdsa::Signature
- SchnorrSig -> schnorr::Signature
- EcdsaSigError -> ecdsa::Error
- SchnorrSigError -> schnorr::Error
- InvalidSchnorrSigSize -> InvalidSignatureSize (this is an error enum variant)
ACKs for top commit:
apoelstra:
ACK 2df51dae15
sanket1729:
ACK 2df51dae15
Tree-SHA512: 7cf63d51ed5fdc737cd59767d9bb96b1e3501634e3aee855493f6a51ad5c5397ce4b25c77f9929abd70d6ceb351fc6520e835da108f4c9a46df5b9c2b52ca6b3