SI-6145 lax typing of args to synthetic case-labels

Use wildcard for the expected type of the arguments of a jump to a label
synthesized by the pattern matcher... except during erasure: we must take the
expected type into account as it drives the insertion of casts!

It's ok since we're typing the translation of well-typed code. The only "type
errors" we catch are skolem mismatches. (after erasure the existential types that
before caused problems have disappeared.)

It's necessary to balance GADT magic, SI-6145, CPS type-driven transforms and
other existential trickiness. I've exhausted all other semi-clean approaches I
could think of:
  - the right thing to do -- packing existential types -- runs into limitations
    in subtyping existential types,
  - casting breaks SI-6145 (and it's an unnecessary cast at run time),
  - not casting breaks GADT typing as it requires sneaking ill-typed trees
    past typer

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