util: correctly inspect Map/Set Iterators

Previously, a MapIterator or SetIterator would
not be inspected properly. This change makes it possible
to inspect them by creating a Debug Mirror and previewing
the iterators to not consume the actual iterator that
we are trying to inspect.

This change also adds a node_util binding that uses
v8's Value::IsSetIterator and Value::IsMapIterator
to verify that the values passed in are actual iterators.

Fixes: https://github.com/nodejs/node/issues/3107
PR-URL: https://github.com/nodejs/node/pull/3119


Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>