util.roles: Add a :policies() method to iterate through available policies
We don't expose the policies directly, to force people to go through :may().
However, there are times when we really just need to know what policies a
role has inside it (e.g. for reporting or debugging purposes).
Structure of a stanza:
stanza {
--- properties ---
tags -- array of tags
--- static methods ---
iq(attrs) --
--- read-only methods ---
reply -- return new stanza with attributes of current stanza
child_with_name(string name) -- return the first child of the current tag with the matching name
--- write methods ---
tag(name, sttrs) -- create a new child of the current tag, and set the child as current
up() -- move to the parent of the current tag
text(string) -- append a new text node to the current tag
}