Add type_id() method to AnyUserData

[nowakf]

I've been messing around with constructing things at runtime, and the problem is, sometimes you don't know what type it is on the rust side. So AnyUserdata::is::() doesn't help you.

Could we add something like:

    pub fn type_id(&self) -> Result<TypeId> {
        let lua = self.0.lua.lock();
        let id = unsafe { lua.get_userdata_ref_type_id(&self.0) };
        id?.ok_or(Error::UserDataTypeMismatch)
    }

to the AnyUserdata impl?

[khvzak]

You can store type id in a metatable during userdata construction, to fetch it back later.
I'll check if I can add AnyUserData::type_id that does not refer to a private container

[nowakf]

You can store type id in a metatable during userdata construction, to fetch it back later.

This is what I was doing: the problem is if you're creating lua methods that take your type as an argument, then it either has to implement FromLuaMulti, or you have to do a bit of boilerplate to convert it back from AnyUserData on every method. And you can't implement FromLuaMulti so it stores the TypeId, because it returns Self.

[khvzak]

Added