[tx] Handle :db.alter/attribute in transactor

[ncalexan]

Right now, the transactor doesn't allow to modify the schema. After #294 (which itself depends on #283), this will be ready for action. This ticket tracks:

• recognizing and acting on :db.alter/attribute assertions;
  This might include allowing certain "safe" data migrations. I.e., converting numeric types; or adding uniqueness constraints; or allowing significant schema changes as long as there's no data in the store; or ...
• updating the relevant metadata structures;
• updating the idents and schema SQL materialized views;
• testing.

@rnewman implemented support for altering the schema in the Clojure implementation in #78 and filed follow-ups in #105. We might just re-purpose at least #105!

[ncalexan]

I see tests in the Clojure implementation around

mentat/test/datomish/schema_changes_test.cljc

Line 212 in 84a80f4

(deftest-db test-alter-schema-cardinality-many-to-one conn

.