Remove redundant 'ElementConstraint'

[effectfully]

ElementConstraint gives some flexibility, but it doesn't seem it's needed. Having the Eq (Element t) constraint for the elem function looks like a sensible prerequisite -- I doubt people would define their own equality type classes and use them instead of Eq. And if elem requires something else than Eq (like Hashable), then this also should be required for constructing such data structure, so you can just constrain the entire instance like this:

instance (Eq v, Hashable v) => Container (HashSet v) where

[chshersh]

@effectfully Thanks a lot for your contribution! This looks like a descent improvement which increases conveniency. Could you also please update CHANGELOG and confirm that benchmarks for elem function work as expected?

[chshersh]

Changelog + benchmarks

[effectfully]

Added a note to the changelog. Unfortunately, I can't check the benchmarks, because I'm constantly getting severely inflated results. Might be a problem with my machine.

[gromakovsky]

1.2.0 is already on Hackage, so this changelog entry is misplaced. It should probably go to 1.3.0. @chshersh?

[chshersh]

@gromakovsky Yes, this should go to 1.3.0. I don't want to bother @effectfully with maintaining changelog properly since it involves knowing specific version policy for the project, etc. I can move this line to 1.3.0 w/o problems but I'm just to lazy to write everything by my own...

[chshersh]

I checked benchmarks by myself. Everything is still working as expected 👌