Add sets package (#7)

**Roadmap**
- [x] Port over desired definitions from Dafny
- [x] Port over desired definitions from Verus
- [x] Port over desired definitions from Why3
- [x] Discuss whether additional definitions must be added to account
for disabling the axiomization in user packages
- [x] Decide whether to use standard libraries of proof assistants like
Coq and Lean
- [x] Restructure package to make it consistent