Add an entity-ref spec for references to entities
Add an entity-ref spec for references to entities

Two examples would be

    (entity-ref 'user)
    (entity-ref 'comment :many? true)

Under the hood, these validate just like

    :workflo.macros.specs.types/ref
    :workflo.macros.specs.types/ref-many

except that by parsing the descriptions of entity-ref specs, we
can obtain the name of the target entity.

workfloapp/app-macros,workfloapp/macros,workfloapp/macros