Drop support for converting unqualified keys

[drgif]

E.g. in a pure Clojure/Cljs/Datomic stack this might not be necessary. For others, metosin/spec-tools transformers might be a better option.

[drgif]

Change the map->entity function to be used in an explicit make-xxx constructor to be defined by library users

• [x] Make creation-date and modified-date entity specs
• [x] Create default ids and dates
• [x] Pass default values to map->entity and merge them in
• [x] Drop support for string uuids (for now anyway)
• [ ] Decide, whether error codes are namespaced keywords or not