The anchor types for [xwt_core].
Intended to be useful in overcoming the restriction on the implementations for foreign types.
That is - if you want to implement something for [xwt_core] but can't -
implement it concretely for an xwt_anchor type and switch to using
xwt_anchor-wrapped types instead of raw xwt_core traits where
applicable.
This implementation is experimental, and we'll see on whether we'll keep maintaining it.