WIP policy_is_permissive, policy_is_restrictive

[iamed2]

Closes #343 (when complete)

This is as far as I got with the time I had today.

Not sure what to make of this error:

===(    1499;0    0/459   0/72     0/1105  117/270  104/408    0... )===psql:test/sql/policy.sql:642: ERROR:  return type mismatch in function declared to return text
DETAIL:  Function's final statement must be SELECT or INSERT/UPDATE/DELETE RETURNING.
CONTEXT:  SQL function "policy_is_restrictive" during startup
test/sql/policy.sql ........... Dubious, test returned 3 (wstat 768, 0x300)

I'm sure I made an error but I'm not sure where.