Unwinding circular dependency chain at request of Simon Urbanek; iterators can now pass R CMD check --as-cran without foreach being present. Also, foreach can now pass its tests when in sequential mode.

git-svn-id: svn://svn.r-forge.r-project.org/svnroot/foreach@25 edb9625f-4e0d-4859-8d74-9fd3b1da38cb