Pull enhancements

[rnewman]

• [x] Implement pulling :db/id, which means we can avoid using rel results in some cases. Rather than [:find [?person (pull ?person [:person/name])] …] we can do [:find [(pull ?person [:db/id :as :person/id :person/name]) ...] …] and handle the returned maps.
• [x] Implement aliases.
• [x] Allow for non-namespaced keywords in StructuredMap. This would allow :as :id instead of :as :something/id.
• [x] Consider whether it's better to return TypedValue::Ref(12345) or {:db/id 12345} — the former is an advantage of our type system, and simplifies extraction of values, but the latter might be more regular when considering fetching more values, and is the way Datomic represents an entity in pull.

• [ ] Single-step pull execution. A query like

  [:find (pull ?x [:a/b :c/d]) :where …]

  can be executed in a single SQL query. The in-progress implementation will take two.

• [ ] Full prepared statement support. This probably looks like inserting the results of the prepared query into a temporary table, and running the pull from there. That makes both queries fixed and allows us to prepare both the 'front' and 'back' of the query.

[ncalexan]

Consider whether it's better to return TypedValue::Ref(12345) or {:db/id 12345} — the former is an advantage of our type system, and simplifies extraction of values, but the latter might be more regular when considering fetching more values, and is the way Datomic represents an entity in pull.

I personally see Datomic's representation as a facet of their execution model -- referentially transparent and generally lazy. Our execution model is so different that I'd only want the Datomic-like representation for consistency of extract results, as you say.

We could, of course, grow :db/id-ref or {:db/id} to disambiguate one case from the other.

[rnewman]

To provide an example, Datomic does this:

(pull ?x [:some/long :some/ref])

➡️

{:some/long 12345 
 :some/ref {:db/id 12345}}

I think we should do this:

{:some/long Binding::Scalar(TypedValue::Long(12345))
 :some/ref Binding::Scalar(TypedValue::Ref(12345))}

and if you want the map form, do this:

(pull ?x [:some/long {:some/ref [:db/id]}])

[rnewman]

Much of this was done in #694 and #682.