Better unsizing, distinguish vtable unsizes

[N1ark]

Adapt the unsizing metadata code, taking inspiration from rustc source. This means we now also support unsizes for e.g. Rc<[T;N]> -> Rc<[T]>.

Rename UnsizingMetadata::VTablePtr -> DirectVTable, to signify an unsize from a non-dyn type into a dyn type (meaning the VTable is statically known!).
This is in contrast to the new NestedVTable, for dyn-dyn casts. In these cases, we translate the ImplExprs from the source and target type, as well as a reindexes field, for whether a reindexing is needed (e.g. dyn T + 'static -> dyn T is a no-op).

[Nadrieril]

For Rc etc, my plan is to add a fake to_unsize method to the CoerceUnsize trait and call that, which would enable composable and polymorphic tracking of what exactly happens, rather than needing a builtin operation that clients have to implement. We'd generate the body of that method for each custom ptr that supports unsizing.

[N1ark]

For Rc etc, my plan is to add a fake to_unsize method to the CoerceUnsize trait and call that, which would enable composable and polymorphic tracking of what exactly happens, rather than needing a builtin operation that clients have to implement. We'd generate the body of that method for each custom ptr that supports unsizing.

hmm as long as that's optional that sounds good :) on a semantics level i'm not fully sure that works without implying any reborrows or loss of region tracking but it could be worth a try for sure