hash.c: same hash value for similar constructs

* hash.c (rb_hash_recursive): make similar (recursive) constructs
  return same hash value.  execute recursively, and rewind to the
  topmost frame with an object which .eql? to the recursive
  object, if recursion is detected.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@43981 b2dd03c8-39d4-4d8f-98ff-823fe69b080e