[m-rev.] for post-commit review: register typeclass_info args more directly

[Zoltan Somogyi]

For review by anyone.

This does make one tiny change in output: if a type variable occurs
in more than one argument type of a typeclass_info, it will be
registered in the rtti_varmaps for its first occurrence, whereas
it used to be registered for its last. However, both should work
equally well (or equally badly, as pointed out by a new XXX
for a very old problem).

Zoltan.
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