Merge #957: Add API to PSBT to enable signing inputs

dd8730e Use new PSBT signing API in example (Tobin C. Harding)
d2367fb Add PSBT sign functionality (Tobin C. Harding)
b80e5ae Re-order import statements (Tobin C. Harding)

Pull request description:

  Add an API for signing inputs to the `PSBT` struct. This is work based on code in `rust-miniscript` and the API design suggestions below from @sanket1729 and @Kixunil.

  Please note, this adds an `unimplemented!` call for taproot inputs. ECDSA signing is complete.

  Includes a patch adding the psbt example from #940 updated to use this new api. Run `cargo run --example psbt --features=bitcoinconsensus` to test it out.

ACKs for top commit:
  dunxen:
    ACK dd8730e
  apoelstra:
    ACK dd8730e
  sanket1729:
    reACK dd8730e

Tree-SHA512: 6345571e53cd3aa4b7ad962536da47ae03ab7c0b088107dc4104676bdb64fcf892e8fa60e0b716f3ef158d88d7058938bf267046721ccf74b2d1b092e9b9aaaa