Add mutation testing

[dunxen]

I've been looking into mutation testing for https://github.com/lightningdevkit/rust-lightning and came across https://github.com/sourcefrog/cargo-mutants which has the advantage of not requiring any actual changes to the source code we need to make. I would not enable it in CI for every PR in general. I'll have to check how long it takes to run on this project. Maybe it is feasible. But if not, then maybe just on merge/scheduled runs.