* enumerator.c: *WIP* Finalize and document Lazy.new. [Bug #7248]
  Add Lazy#to_enum to maintain laziness, and use it for
  methods Enumerable returning Enumerators. [Bug ##7715]
  This makes Lazy#cycle no longer needed, so it was removed.

* test/ruby/test_lazy_enumerator.rb: tests for above