Type abbreviations completed

The initial program is now has bindings, e.g., (Dtabbrev [] "int" (Tapp
[] TC_int)), that replace the functionality of the elaborator. But now
these work in modules too, so, for example, Word8Array.elem is a type
abbreviation for (Tapp [] TC_word8).

metatheory and inferencer soundness build, but nothing else is tested
yet.