Remove SortedSet implementations

[knu]

It required RBTree to perform decently and the external dependency was not suitable for a standard library. The pure ruby fallback implementation was originally meant to be a demonstration of how to write a subclass of Set, and its poor performance was not suitable for use in production.

I decided it should be distributed as an external library instead of bundling it with Set.

[ioquatix]

I completely agree, and it's performance is not great.

[marcandre]

I don't know if these are used much or at all (I notice that there's a typo in the class RestricedSet!), but nevertheless, maybe it would be a good idea to keep compatibility for one full version of Ruby? We could use autoload on both SortedSet and RestricedSet to load a different file that could issue a warning, and remove that for Ruby 3.1. The external library should be available now too (containing the same code) and this would make for a nice transition period for anyone using SortedSet

[knu]

Just pushed sorted_set 1.0.0: https://rubygems.org/gems/sorted_set