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