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
Towards: Missing pickWrapped
Towards
TODO comments
HoleEdit: HoleResult doesn't take focus