A "Coverall" test for uniffi components

This is similar to our examples, but it's intended to be contrived and to ensure we get good test coverage of all possible options.

It's here so it doesn't even need to make a cursory effort to be a "good" example; it intentionally panics, asserts params are certain values, has no-op methods etc. If you're trying to get your head around uniffi then the "examples" directory will be a much better bet.