New issues

Stream: std4

[Matej Penciak]

Hey everyone! After the discussion in today's meeting, I decided to make 3 new issues for std4 that are of varying degrees of involved-ness.

1) Port Lean's HashSets to Std4
2) Port Lean's PersistentHashMap to Std4 (there are other persistent structures as well, so we can additionally do those if we want, but I didn't make it an explicit issue)
3) Related to the short discussion about arrays we should start writing some lemmas about Arrays mirroring those for Lists

I think I roughly ordered the issues from less involved to more involved, so if people want to start claiming them and making branches then feel free! (I think I'm going to make an Arrays branch and start chipping away at the issue after work tomorrow!)