Implement `CoerceUnsized` on all handles

[CraftSpider]

Currently, only a small number of handles can be unsized. Preferably, all handles should be coerce-able. This requires some Rust features though:

• JustMetadata<T>: Allows MetaHandle and OffsetMetaHandle to be unsized - Implement pointee metadata unsizing via a TypedMetadata<T> container rust-lang/rust#97052
• Fallback handle requires unsizing enums. This may be possible to define / implement, just needs a motivating use