[tx] Handle _reverse attribute assertions (back references)

[ncalexan]

An assertion of the form

[[:db/add IDENT :db/_ref-attr ENTID]]

is equivalent to the assertion

[[:db/add ENTID :db/ref-attr IDENT]]

That is, this allows to add back-references. It's particularly useful using the map shorthand of #180: for example, one can add and install a new attribute into the :db.part/db using:

[{:db/id #id-literal[-1]
    :db/ident :my/ident
    :db/valueType :db.type/long
    :db.install/_attribute :db.part/db}]

[ncalexan]

See also #202, which is probably a prerequisite to this work.

[ncalexan]

After #283, this will be ready to roll. Around https://github.com/ncalexan/mentat/blob/6831925af0eafe9e647a462364b6188748f1e840/db/src/tx.rs#L139, you'll need to:

1. recognize that an attribute is reversed;
2. "swap" the e and v values -- this will require some special pattern matching and conversions, and might require type-checking a little earlier than where it happens right now (https://github.com/ncalexan/mentat/blob/6831925af0eafe9e647a462364b6188748f1e840/db/src/tx.rs#L169)
3. add test fixtures asserting the success and some of the failure cases for reversed attributes around https://github.com/ncalexan/mentat/tree/6831925af0eafe9e647a462364b6188748f1e840/tx/fixtures. You might learn from the tests at https://github.com/mozilla/mentat/blob/master/test/datomish/db_test.cljc#L509, although they're not comprehensive.

[ncalexan]

In particular, we should:

• test that reversing an attribute which isn't :db.valueType :db.type/ref fails cleanly (since the resulting e won't be an entid);
• test that we can specify the value v as a numeric entid or a keyword ident;
• think about handling reverse attributes and lookup refs (see [tx] Handle lookup-refs in transactor #183) cleanly -- both the e and v values could be lookup refs.

[ncalexan]

@joe this is a good third ticket in the transactor, following on the work in #180.