code: Check proposal validity in example app

[romac]

In the example app, when we assemble a proposal from its parts, we currently do not check that the signature embedded in the final part (Fin) matches what we expect:

https://github.com/informalsystems/malachite/blob/77c741e19f24e56e0adb0efd150e63087813cb99/code/examples/channel/src/state.rs#L320

The original signature is computed here:

https://github.com/informalsystems/malachite/blob/77c741e19f24e56e0adb0efd150e63087813cb99/code/examples/channel/src/state.rs#L292-L298