[meta] Full Datalog type support (instants, UUIDs, URIs)

Over in #170, I simplified things by not including placeholders for Mentat value types I wasn't going to support at the start: :db.type/instant, :db.type/uuid, and :db.type/uri. (We might consider supporting JSON and a binary blob type like :db.type/bytes as well.)

This ticket tracks supporting these types. Off the top, this will mean:

1. Adding new DB_TYPE_* values to the bootstrapper;
2. Bumping the SQL schema to accommodate the new idents;
3. Adding new ValueType cases;
4. Adding corresponding TypedValue cases;
5. Implementing the conversions to and from SQL;
6. testing the new types in the transactor and potentially in the query engine as well.