Make wallet methods take &mut psbt

Rather than consuming it because that is unergonomic.

src/lib.rs

@@ -158,9 +158,9 @@
 //!     )?;
 //!
 //!     let psbt = "...";
-//!     let psbt = deserialize(&base64::decode(psbt).unwrap())?;
+//!     let mut psbt = deserialize(&base64::decode(psbt).unwrap())?;
 //!
-//!     let (signed_psbt, finalized) = wallet.sign(psbt, None)?;
+//!     let finalized = wallet.sign(&mut psbt, None)?;
 //!
 //!     Ok(())
 //! }