In `layout::CastFrom`, support ZST DSTs

We should be able to treat ZST DSTs as "sized-ish" for the purposes of equivalence. In particular:
- **ZST DST -> Sized** is fine if the source type's "effective size" is equal to the destination type's size
- **Sized -> ZST DST** is fine if the source type's size is equal to the destination type's "effective size"
- **ZST DST -> Unsized** is equivalent to **Sized -> Unsized**
- **Unsized -> ZST DST** is still banned