Enforce policies for transactions

[bplatz]

This closes issue #296. This relies on PR #365 (still waiting approval).

Policy enforcement had been limited to queries. This extends it to now support transactions.

See new tests for a full example. A quick summary is that this policy on products (class :ex/Prouct):

{:id            :ex/ProductPolicy,
:type          [:f/Policy],
:f/targetClass :ex/Product
:f/property    [{:f/path   :schema/name
                 :f/allow  [{:f/targetRole :ex/userRole
                 :f/action [:f/view :f/modify]}]}]}

Would allow a user that is part of:ex/userRole to view and modify :schema/name only - they could neither view nor modify anything else.

Therefore, this transaction would succeed:

{:id          :ex/widget
 :schema/name "Some Product Name"}

And this transaction would fail:

{:id           :ex/widget
 :schema/price 42.99}

[dpetran]

👍