fix: make Concurrent::Set thread-safe on CRuby

Closes ruby-concurrency#900

lib/concurrent-ruby/concurrent/set.rb

@@ -23,9 +23,13 @@ module Concurrent
   # @!macro internal_implementation_note
   SetImplementation = case
                       when Concurrent.on_cruby?
-                        # Because MRI never runs code in parallel, the existing
-                        # non-thread-safe structures should usually work fine.
-                        ::Set
+                        require 'monitor'
+                        require 'concurrent/thread_safe/util/data_structures'
+
+                        class CRubySet < ::Set
+                        end
+                        ThreadSafe::Util.make_synchronized_on_cruby Concurrent::CRubySet
+                        CRubySet
 
                       when Concurrent.on_jruby?
                         require 'jruby/synchronized'

lib/concurrent-ruby/concurrent/thread_safe/util/data_structures.rb

@@ -12,6 +12,25 @@ def self.synchronized(object, &block)
 module Concurrent
   module ThreadSafe
     module Util
+      def self.make_synchronized_on_cruby(klass)
+        klass.class_eval do
+          def initialize(*args, &block)
+            @_monitor = Monitor.new
+            super
+          end
+        end
+
+        klass.superclass.instance_methods(false).each do |method|
+          klass.class_eval <<-RUBY, __FILE__, __LINE__ + 1
+            def #{method}(*args)
+              monitor = @_monitor
+              monitor or raise("BUG: Internal monitor was not properly initialized. Please report this to the concurrent-ruby developers.")
+              monitor.synchronize { super }
+            end
+          RUBY
+        end
+      end
+
       def self.make_synchronized_on_rbx(klass)
         klass.class_eval do
           private