Move core.get_connected_players() implementation to C++

Keeping the ObjectRefs around in a table isn't ideal and this allows
removing the somewhat nonsensical is_player_connected() added in 86ef7147.

src/script/lua_api/l_env.h

@@ -101,6 +101,9 @@ private:
 	// pos = {x=num, y=num, z=num}
 	static int l_add_item(lua_State *L);
 
+	// get_connected_players()
+	static int l_get_connected_players(lua_State *L);
+
 	// get_player_by_name(name)
 	static int l_get_player_by_name(lua_State *L);