Fix `Send` and `Sync` implementations on `Id` and `WeakId`

[madsmtm]

See comments in the code for reasoning.

The impl for Id<T, Shared> is the same as Arc<T>, the impl for Id<T, Owned> is the same as Box<T> and the impl for WeakId is the same as Id<T, Shared> (because you can get the latter from the former).

See this blog post by nyanpasu64 for more information on this.