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

[ncalexan]

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.

[ncalexan]

There are some fledgling tests for SQL conversion (step 5. above) of other TypedValue instances around ncalexan@3e3d740. (They haven't landed quite yet.)

I expect the following encoding:

• :db.type/instant <-> (SQLite INTEGER, tag 4)
• :db.type/uuid <-> (SQLite TEXT, tag 11)
• :db.type/uri <-> (SQLite TEXT, tag 12)

The value type tags above agree with those in the Clojure implementation: see https://github.com/mozilla/mentat/blob/master/src/common/datomish/sqlite_schema.cljc#L246.

Writing real tests isn't really possible until #214 and some follow-ups land, so step 6. above might need a follow-up ticket.

[rnewman]

Filed #426, #427, #431 for this.