Alice to validate Bob's PSBT for correctness

In order for the re-construction of TxLock to be meaningful, we limit
`Message2` to the PSBT instead of the full struct. This is a breaking
change in the network layer.

The PSBT is valid if:

- It has at most two outputs (we allow a change output)
- One of the outputs pays the agreed upon amount to a shared output script

Resolves #260.

swap/src/bitcoin.rs

@@ -14,6 +14,7 @@ pub use crate::bitcoin::redeem::TxRedeem;
 pub use crate::bitcoin::refund::TxRefund;
 pub use crate::bitcoin::timelocks::{BlockHeight, ExpiredTimelocks};
 pub use ::bitcoin::util::amount::Amount;
+pub use ::bitcoin::util::psbt::PartiallySignedTransaction;
 pub use ::bitcoin::{Address, Network, Transaction, Txid};
 pub use ecdsa_fun::adaptor::EncryptedSignature;
 pub use ecdsa_fun::fun::Scalar;