nextmethod simplifications/optimizations, from Tomas Kalibera.


git-svn-id: https://svn.r-project.org/R/trunk@66713 00db46b3-68df-0310-9c12-caf00c1e9a41
NEWS item for r66712

git-svn-id: https://svn.r-project.org/R/trunk@66714 00db46b3-68df-0310-9c12-caf00c1e9a41