This is simply the slice iterator for the dense vector.
- map.values() returns an iterator with references to the values.
- for i in &map iterates over references to the values.
This implements the classic Briggs/Torczon sparse set construct.
Adapt it to our existing EntityRef infrastructure so we can use types
keys instead of just integers like the original paper does.
Also provide a SparseSet<T> type alias which implements a sparse set of
entity refeences. This is actually closer to what the original paper
describes.
