Overload operator -> to return a LuaContext*

Overload operator -> to return a LuaContext*

@rgacogne suggested to return a LuaContext&, but AFAIKS that won't
fly because if the rescriction mentioned in
https://en.cppreference.com/w/cpp/language/operators:

The overload of operator -> must either return a raw pointer, or
return an object (by reference or by value) for which operator ->
is in turn overloaded.

As LuaContext has no -> operator, we must return a raw pointer.