Bring back TxIn and methods on Transaction

[thunderbiscuit]

This PR add the TxIn type from rust bitcoin, and adds the Transaction.input(), Transaction.output(), and Transaction.lock_time() methods.

Description

The input() and output() methods are actually just returning the actual field on the type, hence their name. I don't know why those fields are not called inputs and outputs, but I didn't want to clean up rust-bitcoin's naming convention.

Notes to the reviewers

The LockTime field is a really good one to add in the future, and we should open an issue for it (returning a LockTime instead of a u32).

Changelog notice

Added
  - TxIn type [#536]
  - Transaction.input() method [#536]
  - Transaction.output() method [#536]
  - Transaction.lock_time method [#536]

[#536]: https://github.com/bitcoindevkit/bdk-ffi/pull/536

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

[reez]

ACK 5e3e249

Nice!