Major The value of the "type" keyword should always be a JSON Schema (reference) instead of a qualified type name.
This affects term definitions, parameters, return types, entity sets, singletons
Using $ref to reference functions from function imports and actions from action imports required restructuring actions and functions: these are now objects with one property per function (resp. action) name, and if the function (or action) has overloads, the value is an array of function definitions, otherwise it's a single function definition. See second change to Aggregation vocab for the difference.
This change shows that many of the terms in our standard vocabularies are nullable: we should check whether this makes sense for the term semantics, especially for all of the tagging terms using type Core.Tag.