[tx] Handle lookup-refs in transactor

[ncalexan]

After #164, we parse lookup-refs; this ticket tracks rewriting lookup-refs in assertions. Assertions of the form

[[IDENT :db/ref-attr [:db/unique-attr VALUE]]]

are equivalent to finding the ENTID such that (ENTID :db/unique-attr VALUE) is an assertion, and then transacting [[IDENT :db/ref-attr ENTID]].

In the Clojure implementation, I collected all lookup-refs, wrote them all to a temporary table, and then handled them in a query. This is efficient when lookup up hundreds or thousands of lookup-refs, as happens when re-importing an already imported database. (In this case many lookup-refs match.) It's not efficient when we have only a handful of lookup-refs. A sophisticated implementation might have multiple regimes, sometimes using a temporary table and sometimes using a small bespoke query to do the lookups.

For the record, lookup-refs that don't resolve cause the Mentat transaction to fail (just like Datomic).

[ncalexan]

Per the discussion in #284 (comment), this will be ready for action after #283 lands.

To handle this:

1. Start by understanding http://blog.datomic.com/2014/02/datomic-lookup-refs.html and the section "Lookup refs" in http://docs.datomic.com/identity.html.
2. Add a lookup ref intern set, like we have for tempids, around

   mentat/db/src/tx.rs

   Line 128 in 6831925

   let mut temp_ids = intern_set::InternSet::new();

   .
3. Collect lookup refs from the e and v positions around

   mentat/db/src/tx.rs

   Line 154 in 6831925

   entmod::EntidOrLookupRefOrTempId::LookupRef(_) => {

   and

   mentat/db/src/tx.rs

   Line 164 in 6831925

   bail!(ErrorKind::NotYetImplemented(format!("Transacting lookup-refs is not yet implemented")))

   , respectively.
4. Plumb the intern set through to the (existing) lookup ref resolution code at

   mentat/db/src/tx.rs

   Line 230 in 6831925

   let terms_with_temp_ids = self.resolve_lookup_refs(&lookup_ref_map, terms_with_temp_ids_and_lookup_refs)?;

   .
5. Add tests around https://github.com/ncalexan/mentat/tree/6831925af0eafe9e647a462364b6188748f1e840/tx/fixtures. You might learn from Clojure implementation tests around

   mentat/test/datomish/db_test.cljc

   Line 853 in 84a80f4

   (deftest-db test-lookup-refs conn

   .

[ncalexan]

@joe this is a good fourth (or next) ticket in the transactor. It'll probably be obvious after working through #283, #180 and/or #187.