PushFunction(), CallFunction() and GetReturn() are not to be called independently, but rather only by using the Call() templated overrides.
Push() needs to be left in the public part, it is used for pushing results in the ManualBindings.
Preparation for #418.
This also marginally improves performance, since a strlen() isn't called (inside lua_pushstring()), the string length is stored in the AString object directly.
