Lua的Full UserData、Light UserData和metatable

http://lua.2524044.n2.nabble.com/LightUserData-and-metatables-td3807698.html

"... do you realize that by setting the metatable of a light userdata
you are actually setting the metatable of all light userdata at once ?"
I did not realise this. oops. 

设置Full UserData的metatable设置的是一个Full UserData实例的metatable。

设置Light UserData的metatable设置的是所有Light UserData实例的metatable。