Add back optimizations of HashSet

[julienrf]

Fixes #380 (I checked it locally but didn’t duplicate the test in our repository).

For some reason when I ported the code of HashSet to the new design I only took the minimum amount of code. This PR also takes optimizations from the old collections and put them in the new collections.

[szeiger]

What's with all the private[immutable] methods? Are they supposed to be called from outside of HashSet?

[julienrf]

I copy-pasted from the old collections without making them protected but indeed they should be protected, unless maybe there are some tests somewhere in the old collections that rely on these implementation details?

[julienrf]

I’ve modified the PR to use protected when possible.