Add a new exception class StopIteration, which breaks Kernel#loop
  iteration when raised; backported from 1.9.

* enumerator.c (enumerator_next, enumerator_rewind): Implement
  #next and #rewind using the "generator" library.

* lib/generator.rb: Implement Enumerable::Enumerator#next and
  #rewind.


git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/branches/ruby_1_8@15954 b2dd03c8-39d4-4d8f-98ff-823fe69b080e