More valid schema value types.

[wbrown]

This commit adds more schema value types, even though Datascript doesn't care about the type other than if it's a :db.type/ref or not. The value type keywords follow closely with Datomic.

This allows using a schema defined for Datascript to drive schemas for other databases that care more about type such as Datomic or ElasticSearch.

If it is preferred that :db.type/ref is the only valid type, we could instead have that refer to a def'ed value containing a set of :db.type/ref. This would allow monkey-patching by other modules by redefining the value.

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This change is