Make typechecking unfoldings from interfaces lazier

The old logic was unecessarily strict in loading unfoldings because when
reading the unfolding we would case on the result of attempting to load
the template before commiting to which type of unfolding we were
producing. Hence trying to inspect any of the information about an
unfolding would force the template to be loaded.

This also removes a potentially hard to discover bug where if the
template failed to be typechecked for some reason then we would just not
return an unfolding. Instead we now panic so these bad situations which
should never arise can be identified.

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