feat(gs/achilles): Implement very basic monomorphization

Implement very basic monomorphization, by recording type variable
instantiations when typechecking Call nodes and then using those in a
new hir Visitor trait to copy the body of any generic decls for each
possible set of instantiation of the type variables.

Change-Id: Iab54030973e5d66e2b8bcd074b4cb6c001a90123
Reviewed-on: https://cl.tvl.fyi/c/depot/+/2617
Reviewed-by: glittershark <grfn@gws.fyi>
Tested-by: BuildkiteCI

users/glittershark/achilles/tests/compile.rs

@@ -14,12 +14,11 @@ const FIXTURES: &[Fixture] = &[
         exit_code: 5,
         expected_output: "",
     },
-    // TODO(grfn): needs monomorphization
-    // Fixture {
-    //     name: "functions",
-    //     exit_code: 9,
-    //     expected_output: "",
-    // },
+    Fixture {
+        name: "functions",
+        exit_code: 9,
+        expected_output: "",
+    },
     Fixture {
         name: "externs",
         exit_code: 0,