Note that is still pretty limited. We only permit opaque types to
implement abilities, abilities cannot have type arguments, and also no
other functions may depend on abilities
Turns out that we can't always assume that a successful unification of
type alias type variables means that those aliases had the same real
type from the start. Because type variables may contain unbound type
variables and grow during their unification (for example,
`[InvalidNumStr]a ~ [ListWasEmpty]b` unify to give `[InvalidNumStr,
ListWasEmpty]`), the real type may grow as well.
For this reason, continue to explicitly unify alias real types for now.
We can get away with not having to do so when the type variable
unification causes no changes to the unification tree at all, but we
don't have a great way to detect that right now (maybe snapshots?)
Closes#2583
Previously we only registered record accessor closures in anonymous
contexts, where we assume they must already be specialized based on the
surrounding contexts. This is not true in general since one might bind
an accessor to a name.
Closes#2567
Now, when we have two aliases like
```
T a : [ A, B (U a) ]
U a : [ C, D (T a) ]
```
during the first pass, we simply canonicalize them but add neither to
the scope. This means that `T` will not be instantiated in the
definition of `U`. Only in the second pass, during correction, do we
instantiate both aliases **independently**:
```
T a : [ A, B [ C, D (T a) ] ]
U a : [ C, D [ A, B (U a) ] ]
```
and now we can mark each recursive, individually:
```
T a : [ A, B [ C, D <rec1> ] ] as <rec1>
U a : [ C, D [ A, B <rec2> ] ] as <rec2>
```
This means that the surface types shown to users might be a bit larger,
but it has the benefit that everything needed to understand a layout of
a type in later passes is stored on the type directly, and we don't need
to keep alias mappings.
Since we sort by connected components, this should be complete.
Closes#2458
