lang: Polymorphic lambda functions aren't allowed for now

We don't allow a lambda variable to be polymorphic anymore. Of course
this isn't bad, but it makes things too difficult, at least for now.
It's also likely that we don't need this specific feature very often I
think.

lang/interpret_test/TestAstFunc2/polymorphic-lambda.txtar

@@ -17,5 +17,4 @@ $out2 = $add($val) # hellohello
 
 test fmt.printf("%s + %s is %s", $val, $val, $out2) {} # simple concat
 -- OUTPUT --
-Vertex: test[2 + 2 is 4]
+# err: errUnify: can't unify, invariant illogicality with equality: base kind does not match (Int != Str)
-Vertex: test[hello + hello is hellohello]