Support determinism annotations for functions invoked by Bud?

[neilconway]

If a rule invokes a function, we are currently conservative: we mark the associated operator for rescan at every tick unless the function belongs to a small class of functions known to be safe (e.g., ip_port). We could consider providing a function annotation that would allow users to add new functions to the "safe function" whitelist. (Postgres provides something analogous, BTW -- http://www.postgresql.org/docs/9.2/static/sql-createfunction.html).