Small speedup to reorder. Avoids memcpy call overhead.  Likely 5% improvement,  but could be up to 50% if data is already partially sorted or clustered.

git-svn-id: svn+ssh://svn.r-forge.r-project.org/svnroot/datatable@1072 edb9625f-4e0d-4859-8d74-9fd3b1da38cb
Merge branch 'refs/heads/r-forge-git-svn-local'