A drop-in replacement for BTreeMap, where the keys have the property of being array indices.
Some values may be missing. Much more efficient at operations on individual objects, because
it just becomes a simple array lookup.
Use with FixedMap. From a particular key, extract a usize. These values should be
roughly contiguous; the space used by the FixedMap will be O(n) with respect to the largest
value returned here.