remove rayon from hashbrown dep

[lthiery]

When trying to use h3ron v0.12.0 without any features, rayon still gets pulled in. This is because hasbrown now depends on it as a feature, but it appears it might be unnecessary?

[lthiery]

Nevermind - looking more closely it does look like you lose the parallel iterator without this flag. It might be cool to do something like this to make rayon an option for h3ron clients. But I'll try that another time perhaps.

[nmandery]

Thanks for bringing this up - I had not noticed this. The activated hashbrown features where generally in conflict with the h3ron featuregates. 65abf80 should fix this.

[lthiery]

Thanks for bringing this up - I had not noticed this. The activated hashbrown features where generally in conflict with the h3ron featuregates. 65abf80 should fix this.

Thanks for fixing this up!