Use Amount in TxOut

[rustaceanrob]

Mirrors the Rust API and enforces type safety over FFI.

Closes #774

Changelog notice

• Use Amount in TxOut

Checklists

All Submissions:

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

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 ca0b6bb

Tested locally and also tested w swift bindings + iOS app