Efficient filtered functions in Evd. We test that a filter is actually
different from the identity before trying to compute filtered env/hyps.

According to profiling, it seems that the filter operation is NEVER taken,
that is, all filters that reach toplevel are dummy. There is surely something
to do here...

git-svn-id: svn+ssh://scm.gforge.inria.fr/svn/coq/trunk@16960 85f007b7-540e-0410-9357-904b9bb8a0f7