Efficient maps of integer id keys to values, backed by an underlying Vec.

• Compiles on stable rust
  • I only officially support the latest stable ;)
• Automatically derived IntegerId for enums and newtype structs
  • Implemented in the idmap-derive proc_macro crate
• Maintains insertion order of the entries, as there's an indirection like OrderMap.
  • Therefore, entries which aren't present take little space, as only a u32 needs to be stored.
  • This indirection can be avoided with a DirectIdMap which doesn't preserve order, and saves space when the ids of the map's keys are densly packed and mostly present.