3263: Implement unsizing coercions using Chalk r=matklad a=flodiebold
These are coercions like `&[T; n] -> &[T]`, which are handled by the `Unsize` and `CoerceUnsized` traits. The impls for `Unsize` are all built in to the compiler and require special handling, so we need to provide them to Chalk.
This adds the following `Unsize` impls:
- `Unsize<[T]> for [T; _]`
- `Unsize<dyn Trait> for T where T: Trait`
- `Unsize<dyn SuperTrait> for dyn SubTrait`
Hence we are still missing the 'unsizing the last field of a generic struct' case.
Co-authored-by: Florian Diebold <florian.diebold@freiheit.com>
Co-authored-by: Florian Diebold <flodiebold@gmail.com>
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