* enum.c (enum_slice_when): New method: Enumerable#slice_when.
  (slicewhen_i): New function.
  (slicewhen_ii): New function.

* enumerator.c (InitVM_Enumerator): New method:
  Enumerator::Lazy#slice_when.

  [ruby-core:62499] [Feature #9826]



git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@47652 b2dd03c8-39d4-4d8f-98ff-823fe69b080e