Remove dependency on bitcoin-ffi

[thunderbiscuit]

This PR, one commit at a time, brings the external types from bitcoin-ffi 0.1.2 back into our own bitcoin.rs source file.

WIP.

It builds on #672, so must be merge after.

Description

Notes to the reviewers

Changelog notice

Checklists

All Submissions:

• I've signed all my commits
• I followed the contribution guidelines
• I ran cargo fmt and cargo clippy before committing

New Features:

• I've added tests for the new feature
• I've added docs for the new feature

Bugfixes:

• This pull request breaks the existing API
• I've added tests to reproduce the issue which are now passing
• I'm linking the issue being fixed by this PR

[thunderbiscuit]

I'm not sure why this PR on GitHub keeps the commits from #672, but it makes it messy to review. Replacing with #674.