Make record type definitions LaTeX-print more nicely

There is a trace that can be altered to get a slightly different style
if desired. Both styles demonstrated in the dupop20171208
test-case (look at the _expected file).

Refine type variables printed by EmitTeX in record declarations

Now those appearing in the record type on the LHS (e.g., ('a,'c) foo),
and the fields in the type actually match up. Adjusted test-cases to
match. Pleasantly, the existing behaviour of prim_mk_const and the
datatype machinery preserves the type variables as the user gave them.

Fix TeX p/printing bug caused by having multiple types of same name

If the types had different arities, mungers would produce strange
mk_type exceptions.