Publish the rust-bitcoin types in their own crate

[thunderbiscuit]

The types that reside in the current bitcoin.rs file are the ones that come from the rust-bitcoin crate. I have made good progress to pull those out of this repo and allow them to live in a separate crate and simply be imported. This has serious advantages in the long run:

• It allows other bitcoin projects that build language bindings using uniffi to leverage these types without re-writing them and maintaining them themselves
• If other projects also use those types, it brings more eyes and review on the codebase
• We potentially pool the burden of development and exposing of more rust-bitcoin types among more projects