Use Psbt instead of PSBT

Use Psbt instead of PSBT

Idiomatic Rust uses lowercase for acronyms for all characters after the
first e.g. `std::net::TcpStream`. PSBT (Partially Signed Bitcoin
Transaction) should be rendered `Psbt` in Rust code if we want to write
idiomatic Rust.

Use `Psbt` instead of `PSBT` when aliasing the import of
`PartiallySignedTransaction` from `bitcoin` library.