Add IfaceType deduplication table to interface file serialisation

Although we do not really need it in the interface file serialisation, as the deserialisation uses getWithUserData, we need to mirror the structure getWithUserData expects. Thus, we write essentially an empty IfaceType table at the end of the file, as the interface file doesn't reference IfaceType.

Sibling PR to ghc!12371 (closed)

assigned to @fendor

mentioned in merge request !58 (closed)

merged