Done as part of flattening util.
Currently in `util` module we have a bunch of modules that provide
cryptography related functionality.
Create a `crypto` module and move into it the following:
- ecdsa
- schnorr
- key
To improve uniformity and ergonomics, do the following re-names while we
are at it:
- EcdsaSig -> ecdsa::Signature
- SchnorrSig -> schnorr::Signature
- EcdsaSigError -> ecdsa::Error
- SchnorrSigError -> schnorr::Error
- InvalidSchnorrSigSize -> InvalidSignatureSize (this is an error enum variant)
Tobin C. Harding