Refactoring schnorrsig

[dr-orlovsky]

As a part of rust-bitcoin/rust-bitcoin#588 discussion I propose this form of refactoring for existing multiple key and signature types:

1. All keys, including BIP-340 specific, are moved into key module
2. schnorrsig renamed to schnorr to have more reasonable schnorr::Signature name
3. Old (non-schnorr) signatures are extracted into new ecdsa module, such that we can always prefix specific signature with proper type name (ecdsa::Signature and schnorr::Signature).

I understand that PR may be hard to review due to a big size of the diff, so I look for concept ACK and suggestions on how to simplify the review process

[real-or-random]

Are you aware of #327 ?

[dr-orlovsky]

@real-or-random sorry, I missed that. Feel free to close this one if that one is considered the route forward

[real-or-random]

Well, I think it's not yet decided (even if we want to do something at all), but now we have two suggestions. :)

[thomaseizinger]

I think the proposal is pretty much the same :)
In #327, I also changed around the names of the functions on Secp256k1 and a few other, minor things.

I don't mind much which of these two moves forward as long as we can land some improvement here!

[dr-orlovsky]

I think #327 is better structured in terms of commit structure, has things on top (like improving function names) and more reviews already. Nice thing about mine is that it confirms that the approach taken by @thomaseizinger can be independently reproduced and such changes are needed. The fact that we did almost the same PRs independently is a good showcase that the new API design is more intuitive.

I will move this PR to a draft mode for now, such that reviewers time is not wasted on this one - and to have a comparison PR which may simplify reviewing / confirmation of the correctness of the other PR (by git difff <two branches> or something like that)