Destructuring mutable references

[WaDelma]

This allows getting mutable reference into the destructured object.

Here is an use case for it that I think is especially confusing if you don't know what it's about:
https://github.com/VoxWave/Mato/blob/6fa4a1a7f5bcad442612032b297272be7d3b6f32/src/main.rs#L64