eolian: introduce the keyword required
This introduces a new keyword called required. It only works on mixins.
You can specify a list of regular/abstract classes in there.
Classes specified after the required keyword are later used to verify
the usage of the mixin. With this feature a mixin can define a list of
types that the inheriting object (the object that inherits from a mixin)
needs to fullfill, if one class that is required is not in the
implemented classes, then eolian will bail out.
Differential Revision: https://phab.enlightenment.org/D7584