A map data structure backed by a vector. The map is guaranteed not to contain duplicate keys, but entries
are not sorted by key–entries are included in insertion order.
All operations are O(N) in the size of the map–the intention of this data structure is only to provide
the convenience of programming against a map API.
Large maps should use handwritten parent/child relationships instead.
Maps that need sorted iteration rather than insertion order iteration should also be handwritten.