lang: unification: Don't stall the solver over generators

If we have a solution, and all that remains are generators, then feel
free to remove them and win.

lang/interpret_test/TestAstFunc2/printfsimple0.output

@@ -0,0 +1 @@
+Vertex: test[the answer is 42]

lang/interpret_test/TestAstFunc2/printfsimple0/main.mcl

@@ -0,0 +1,5 @@
+import "fmt"
+
+$out = 42
+
+test fmt.printf("the answer is %d", $out) {}