This is my first piece of Rust code. I wrote it partly to learn how to work with the heap in Rust, and partly because I needed these for another project.
ThinVec is very generic and compares quite well with Vec. It's a tiny bit slower on push if you're just using it by itself, but it quickly becomes better than Vec if it's used inside another data structure (e.g. Vec<Vec<_>> or Map<_, Vec<_>>).
ThinMap and ThinSet are special purpose, mostly for primitives, or small key/values. If used as such, they are much better than HashMap/HashSet, with no downsides.
V64 is very special purpose. If you have vectors whose total population is expected to fit in 7 bytes, it's much faster than Vec, but otherwise, it takes about a 2x hit on push for large vectors (which is not where it's meant to be used).