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, 1 insertions, 0 deletions

diff --git a/defs/id.def b/defs/id.def
index 8df6cf12e2..097e34e405 100644
--- a/defs/id.def
+++ b/defs/id.def
@@ -7,6 +7,7 @@ firstline, predefined = __LINE__+1, %[\
   inspect
   intern
   object_id
+  const_added
   const_missing
   method_missing                                        MethodMissing
   method_added