Fix "" + list_expr producing list_expr

The type checks were not strong enough. While on it, strengthen other similar checks as well.

Reported by @Tonaie. Fixes #13.

unit_tests/regression.suite/issue-13.out

@@ -0,0 +1,10 @@
+default
+{
+    state_entry()
+    {
+        llParticleSystem("" + llGetPhysicsMaterial());
+        llParticleSystem("" + llGetPhysicsMaterial());
+        llParticleSystem(0 + llGetPhysicsMaterial());
+        llParticleSystem(0 + llGetPhysicsMaterial());
+    }
+}