feat(loans): add mutation testing decorations

[daniel-savu]

Decorates all functions in the loans pallet with #[cfg_attr(test, mutate)], so mutagen can mutate them and check if the unit tests break, "killing" the mutations. Only unit tests are used when checking, so some of the surviving mutants exist because they're tested in an integration test.

Most surviving mutants just remove the deposit_event call, however some logic in the interest rate model should actually be better tested. This is the output about surviving mutants: SURVIVED.txt

The mutagen dependency uses the latest master on their GitHub, because version 2.x wasn't released to crates.io for some reason.