Add a Module#const_added callback

[Feature #17881]

Works similarly to `method_added` but for constants.

```ruby
Foo::BAR = 42 # call Foo.const_added(:FOO)
class Foo::Baz; end # call Foo.const_added(:Baz)
Foo.autoload(:Something, "path") # call Foo.const_added(:Something)
```

1 files changed, 39 insertions, 0 deletions

diff --git a/test/ruby/test_module.rb b/test/ruby/test_module.rb
index 9368940050..0a6959c5e6 100644
--- a/test/ruby/test_module.rb
+++ b/test/ruby/test_module.rb
@@ -1675,6 +1675,45 @@ class TestModule < Test::Unit::TestCase
     assert_match(/::X\u{df}:/, c.new.to_s)
   end
 
+
+  def test_const_added
+    eval(<<~RUBY)
+      module TestConstAdded
+        @memo = []
+        class << self
+          attr_accessor :memo
+
+          def const_added(sym)
+            memo << sym
+          end
+        end
+        CONST = 1
+        module SubModule
+        end
+
+        class SubClass
+        end
+      end
+      TestConstAdded::OUTSIDE_CONST = 2
+      module TestConstAdded::OutsideSubModule; end
+      class TestConstAdded::OutsideSubClass; end
+    RUBY
+    TestConstAdded.const_set(:CONST_SET, 3)
+    assert_equal [
+      :CONST,
+      :SubModule,
+      :SubClass,
+      :OUTSIDE_CONST,
+      :OutsideSubModule,
+      :OutsideSubClass,
+      :CONST_SET,
+    ], TestConstAdded.memo
+  ensure
+    if self.class.const_defined? :TestConstAdded
+      self.class.send(:remove_const, :TestConstAdded)
+    end
+  end
+
   def test_method_added
     memo = []
     mod = Module.new do