This change also means we must update the interface of `Dict.empty` and
`Set.empty` from
```
Dict.empty : Dict k v
```
to
```
Dict.empty : {} -> Dict k v
```
Makes sure that we turn on `debug-symbols` in mono tests, and that this
feature is fully respected in symbol generation, so that output in
release + debug builds are the same.
Closes#4435
Even if there are no changes to alias arguments, and no new variables were
introduced, we may still need to unify the "actual types" of the alias or opaque!
The unification is not necessary from a types perspective (and in fact, we may want
to disable it for `roc check` later on), but it is necessary for the monomorphizer,
which expects identical types to be reflected in the same variable.
As a concrete example, consider the unification of two opaques
P := [Zero, Succ P]
(@P (Succ n)) ~ (@P (Succ o))
`P` has no arguments, and unification of the surface of `P` introduces nothing new.
But if we do not unify the types of `n` and `o`, which are recursion variables, they
will remain disjoint! Currently, the implication of this is that they will be seen
to have separate recursive memory layouts in the monomorphizer - which is no good
for our compilation model.
Closes#3653
