Change TxBuilder.finish() to return new TxBuilderResult

[notmandatory]

Description

Change TxBuilder.finish() to return new TxBuilderResult.

Notes to the reviewers

This fixes #179 in that it return both PartiallySignedBitcoinTransaction and TransactionDetails encapsulated in a new TxBuilderResult structure. It does not calculate the fee rate which requires #208.

Changelog notice

• Breaking Changes
  • Changed TxBuilder.finish() to return new TxBuilderResult.
• APIs Added
  • Added TxBuilderResult with PSBT and TransactionDetails.

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

[notmandatory]

If this looks OK should we try to get this into 0.10.0? or wait for 0.11.0 when we should also have the FeeRate calculation?

[thunderbiscuit]

ACK fadb316.