This seemed to be an oversight seeing as DenseMap has these conversions.
This patch does the following:
- Adds a default constructor to the iterators.
- Allows DenseSet::ConstIterators to be copy constructed from DenseSet::Iterators
- Allows mutual comparison between Iterators and ConstIterators.
All of these are available in the DenseMap implementation, so the implementation here is trivial.
Spurious ;