Add type abbreviations to the concrete syntax.

Next step will be to parse them (i.e., parsing completeness will fail
at the moment).
Refine PEG to handle type abbreviations

Prove soundness, completeness, and that ptree conversion is total.

Note in passing that semantics/cmlPtreeConversion is not converting a
parse-tree corresponding to the "bool" type-operator to TC_bool, but
rather to

   TC_name (Short "bool")

Also, the list of possible tctors is missing TC_list.