I started implementing a generic multiset that I could use for problems since rust std lacks one. I have an initial implementation for it: https://github.com/satylogin/cp-lib/blob/main/src/ds/multiset.rs
I will keep on adding methods as and when they are needed. Sharing here in case someone needs it. In case you do decide to use it and find some bugs, or have a feature request, do let me know.