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

[thunderbiscuit]

I have be quite successful at migrating Kotlin and Python to this temporary crate. You can see the PR here.

My current issue is around how to build the Swift package in a way that types from the bdk library can see and use the types from the rust-bitcoin library.

[thunderbiscuit]

This will be solved once #546 is merged.