Lamdu.Data.Definition: Add Definition with payload as analog of Expression with payload

Renames:
Definition.Body -> Definition.Content
Definition.Definition -> Definition.Body

Return Definition.Definition from ExprIRef.Load. Take ditto in Sugar.convertDefI
Sugar.Convert.Hole: forked prefix -> f prefix
ExprIRef: Add addProperties
Sugar.Convert.Hole/HoleEdit: Hole results are immediately written, and not only when picked

Hole results event maps usable from active hole result.

Widget.EventResult is a Monoid