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/protocol/bob/state.rs

@@ -188,7 +188,7 @@ pub struct State1 {
 impl State1 {
     pub fn next_message(&self) -> Message2 {
         Message2 {
-            tx_lock: self.tx_lock.clone(),
+            psbt: self.tx_lock.clone().into(),
         }
     }