Multi_bimap - many-to-many bimap freely made of two arbitrary maps

The popular Rust crate for bidirectional maps, bimap, is limited to one-to-one relations (bijections) only (and appears unmaintained). But sometimes you need to have one-to-many and many-to-many relations, or have them made of arbitrary containers instead of having to always use either BTreeMap or HashMap. I couldn't find anything for that, so I wrote my own Rust crate, multi_bimap: It's fully generic over its underlying containers (two antiparallel maps of sets), so you can have an arbitrar...