In Ruby 2, SortedSet was part of the standard library. In Ruby 3, it
was removed from stdlib and put into a gem. This new gem implements
SortedSet using RBTree (from the 'rbtree' gem). As a result, the
behavior of `SortedSet#eql?` has changed; two SortedSets only compare
as equal using `#eql?` if they are the same object, not if their
contents are the same.

This behavior does not match built-in Ruby data structures like `Hash`
and is arguably incorrect. It was reported as an issue in the GitHub
repository for `sorted_set` almost a year ago, but has not been fixed.

We would love to see this issue with the new `sorted_set` gem fixed,
but in the meantime, we can simply avoid using `SortedSet#eql?` for
testing equality in our specs.

The behavior of `&:method` blocks has changed in Ruby 3.0. Previously
they had arity -1; now it's -2. I don't know the reason for this
change.

While we fix that, we can also be more strict in rejecting blocks with
an arity which we can't use. These would result in an exception
anyways, but it will be better to raise an exception with a more
understandable error message.

It seems that the behavior of `Kernel#loop` might have changed a bit
in Ruby 3.0. Adjust `SortedSet#eql?` accordingly.

Ruby's SortedSet used to be included in set.rb (standard library).
Starting from Ruby 3.0, it's not. Rather, it has been broken out into a
gem.

So to keep the one part of `Immutable` which references SortedSet
working, we need to pull in that gem. I don't like this, but it's
better than potentially breaking user code.

In the commit message for 952d11a, I wrongly suggested that the code
for this method needed to be adjusted for Ruby 3.0. It seems that
rather, I had broken it when refactoring uses of `while true` loops to
use the `loop` method instead.

With that change, the final `rescue` clause became dead code.

…ng each other

Thanks to Sean Collins for reporting that he was getting the following
warning when using Immutable::Hash:

    immutable-ruby/lib/immutable/vector.rb:2: warning: immutable-ruby/lib/immutable/vector.rb:2: warning: loading in progress, circular require considered harmful - immutable-ruby/lib/immutable/hash.rb

To break the loop, move the definitions of Immutable::Hash,
Immutable::Vector, and Immutable::Set (all of which depend on each
other) into a new file, which is required from hash.rb, vector.rb, and
set.rb, but which doesn't require any of those files itself.