Possible use-after-free with `proplist::Iterator`

Affected versions contained a possible use-after-free issue with property list iteration
due to a lack of a lifetime constraint tying the lifetime of a proplist::Iterator to the
Proplist object for which it was created. This made it possible for users, without
experiencing a compiler error/warning, to destroy the Proplist object before the iterator,
thus destroying the underlying C object the iterator works upon, before the iterator may be
finished with it.

This impacts all versions of the crate before 2.5.0 back to 1.0.5. Before version
1.0.5 the function that produces the iterator was broken to the point of being useless.