implement iter() and utility methods

[nicolaskagami]

What does this PR add?

• IntoIter / Iter / IterMut
• keys()
• values() / values_mut()
• len() and is_empty()
• get() / get_mut()
• retain()

closes #3

Details

• In order to support iteration, each entries map now stores ((K, u8), ValueIndex) instead of just ValueIndex. This had an impact of roughly 1.5% extra time on insert benchmarks. This was deemed worthwhile since it did not affect the lookup time and enables a very easy iter implementation.

• The trie's internal structure guarantees lexicographic order of (len, key) for iterators basically for free.

• Currently, IterMut requires unsafe to produce &'a mut V references. This is because values are stored in a contiguous Vec<V> for lookup cache locality, as opposed to keeping them in the BTreeMap where this would be trivial.

  • An alternative (I think) would be to collect &mut references into a Vec<Option<&mut V>> upfront and hand them out one by one, avoiding unsafe at the cost of an allocation and an extra indirection on every call to next(), which was judged not to be worth it.