Inline polymorphic calls at usage sites

This is a bit.. ugly, or at least seems suboptimal, but I can't think of
a better way to do it currently aside from demanding a uniform
representation, which we probably don't want to do.

Another option is something like the defunctionalization we perform
today, except also capturing potential uses of nested functions in the
closure tag of an encompassing lambda. So for example,

```
f = \x -> \y -> 1
```

would now record a lambdaset with the data `[Test.f
[TypeOfInnerClos1]]`, where `TypeOfInnerClos1` is e.g.
`[Test.f.innerClos1 I8, Test.f.innerClos1 I16]`, symbolizing that the
inner closure may be specialized to take an I8 or I16. Then at the time
that we create the capture set for `f`, we create a tag noting what
specialization should be used for the inner closure, and apply the
current defunctionalization algorithm. So effectively, the type of the
inner closure becomes a capture.

I'm not sure if this is any better, or if it has more problems.
@folkertdev any thoughts?

Closes #2322

compiler/test_mono/src/tests.rs

@@ -1238,6 +1238,19 @@ fn monomorphized_applied_tag() {
     )
 }
 
+#[mono_test]
+fn aliased_polymorphic_closure() {
+    indoc!(
+        r#"
+        n : U8
+        n = 1
+        f = \{} -> (\a -> n)
+        g = f {}
+        g {}
+        "#
+    )
+}
+
 // #[ignore]
 // #[mono_test]
 // fn static_str_closure() {