Add `sort_skip`

Can be used to keep primary-key attributes out of `sorted_by_epk`
Fix setup of `MOM.Attr._A_Entity_.P_Type`

The `P_Type` of a `MOM.Attr._A_Entity_` attribute can be specified
either by a reference to the`Bare Essence` or by the name of the E_Type.

The meta machinery must set `P_Type` to the appropriate app-type
specific E_Type. Up to now, this was handled in multiple places and some
cases didn't work, failing with weird tracebacks.

Now, `MOM.Attr._A_Entity_.__init__` does the right thing.