Add Transaction struct, update PSBT and Blockchain to use it

[notmandatory]

Description

Add new Transaction structure that can be created from transaction consensus encoded bytes. Update PartiallySignedTransaction.extract_tx() to return a Transaction instead of a the transaction bytes. Update Blockchain.broadcast() to take a Transaction parameter.

Notes to the reviewers

Fixes #157.

Changelog notice

Added

• New Transaction structure that can be created from or serialized to consensus encoded bytes.

Changed

• PartiallySignedTransaction.extract_tx() returns a Transaction instead of a the transaction bytes.
• Blockchain.broadcast() takes a Transaction instead of a PartiallySignedTransaction.

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

[notmandatory]

I marked this as ready to review, it's rebased and I don't expect any more code changes. But I still need to do manual testing to confirm the transaction broadcast.

[thunderbiscuit]

Tested ACK 8e54ada. I ran a script using bdk-jvm and your test vector. Looks good! Another win for the LDK/BDK integration.