Use bitcoin::secp256k1 over secp256k1_zkp

[bennyhodl]

As a pre-req to #221

I don't see a specific reason for using secp25k1_zkp

[Tibo-lg]

secp256k1_zkp is required to use the ecdsa_adaptor module so I don't think it's possible to use just secp256k1.

[bennyhodl]

I think it would be worth while to convert everything besides the adaptor stuff. Ran into a lot of dependency issues when upgrading ₿itcoin with #223

[Tibo-lg]

I don't think it's possible, the adaptor signature module uses the types from secp256k1 so if they're not from the same version it won't work.

[bennyhodl]

Made a PR #226