generalize unique.default() to automatically work for more S3 classes


git-svn-id: https://svn.r-project.org/R/trunk@66773 00db46b3-68df-0310-9c12-caf00c1e9a41
A couple of not yet used (ifdef`ed), comments, and cosmetics

git-svn-id: https://svn.r-project.org/R/trunk@66774 00db46b3-68df-0310-9c12-caf00c1e9a41