Substitution of a pack expansion type may now produce a pack type.
We immediately expand that pack when transforming a tuple, a function
parameter, or a pack.
I had to duplicate the component-wise transformation logic in the
simplifyType transform, which I'm not pleased about, but a little
code duplication seemed a lot better than trying to unify the code
in two very different places.
I think we're very close to being able to assert that pack expansion
shapes are either pack archetypes or pack parameters; unfortunately,
the pack matchers intentionally produce expansions of packs, and I
didn't want to add that to an already-large patch.
There are a lot of problems caused by our highly-abstract substitution
subsystem. Most of them would be solved by a more semantic / holistic
understanding of the active transformation, but that's difficult to do
because we just pass around function_refs. The first step in fixing
that is to pass around a better currency type. For now, it can just
hold the function_refs (and the SubstOptions).
I've set it up so that the places that just apply SubstitutionMaps
are constructing the IFS in a standard way; that should make it easy
to change those places in the future.
