Clean up push_instance_metatable().
Clean up push_instance_metatable().

Do not create unused boolean field "__luabind_class".
Use lua_rawset() + lua_pushliteral() instead of the slower lua_setfield().
Use lua_createtable() with proper arguments instead of lua_newtable(): should
give better performance.
Use lua_pushcfunction() instead of lua_pushcclosure() with third argument set
to 0.
Do not unnecessarily clear the stack before calling lua_error().

Oberon00/luabind,Oberon00/luabind,lonski/luabind