count extant `IStr`s

[ickk]

It may be possible to use an atomic counter to track how many copies of an IStr currently exist, allowing the interner to recover memory when all copies have been dropped.

• Would have to remove Copy, since Clone impl would increment said counter.
• Would have to change IStr::as_str and IStr::as_c_str methods to return non-'static references, since they could no-longer out-live the IStr (without some other way to first permanently-leak it)