Enhance `HashSet` API
Summary:
Some improvements to the `HashSet` API:
* Make `create` take `unit` instead of `int`. Callers who want to create a fixed-size hashset have been ported to the new `of_list` constructor.
* Add labels to function parameters.
* Add `to_list`/`of_list` functions.
* Add a handful of utility functions (`union`, `intersect`, `filter`).
Also migrate to use `Core_kernel` internally.
Most of this commit is updating call-sites.
Differential Revision:
D20269911
fbshipit-source-id:
b2f1c87e6560d1c1afc02553021f869ce631df72