Type variables can only be used on functions (and in number literals as
a carve-out for now). In all other cases, a type variable takes on a
single, concrete type based on later usages. This check emits errors
when this is violated.
The implementation is to check the rank of a variable after it could be
generalized. If the variable is not generalized but annotated as a type
variable, emit an error.
Sometimes, we might need to fixpoint-fix a unification like
[ Bar [ Bar <a>, Foo ], Foo ] as <a> 🛠️ [ Bar <b>, Foo ] as <b>
where we hit a comparison between <a> and <b>. In this case, follow each
recursion point independently and see if we can find the chain to the needle
we were searching for.
Closes#5476
Closes#3224
This should also fix a number of other issues often observed when a
lambda is never actually called, but we fail to codegen because it has
an empty lambda set.
