Import set 1.0.0

- SortedSet has been removed for dependency and performance reasons.
- Set#join is added as a shorthand for `.to_a.join`.
- Set#<=> is added.

https://github.com/ruby/set/blob/v1.0.0/CHANGELOG.md

test/test_sorted_set.rb

@@ -0,0 +1,45 @@
+# frozen_string_literal: false
+require 'test/unit'
+require 'set'
+
+class TC_SortedSet < Test::Unit::TestCase
+  def base_dir
+    "#{__dir__}/../lib"
+  end
+
+  def assert_runs(ruby, options: nil)
+    options = ['-I', base_dir, *options]
+    r = system(RbConfig.ruby, *options, '-e', ruby)
+    assert(r)
+  end
+
+  def test_error
+    assert_runs <<~RUBY
+      require "set"
+
+      r = begin
+        puts SortedSet.new
+      rescue Exception => e
+        e.message
+      end
+      raise r unless r.match? /has been extracted/
+    RUBY
+  end
+
+  def test_ok_with_gem
+    assert_runs <<~RUBY, options: ['-I', "#{__dir__}/fixtures/fake_sorted_set_gem"]
+      require "set"
+
+      var = SortedSet.new.to_s
+    RUBY
+  end
+
+  def test_ok_require
+    assert_runs <<~RUBY, options: ['-I', "#{__dir__}/fixtures/fake_sorted_set_gem"]
+      require "set"
+      require "sorted_set"
+
+      var = SortedSet.new.to_s
+    RUBY
+  end
+end