better IStream.concat

git-svn-id: svn+ssh://scm.gforge.inria.fr/svn/coq/trunk@17064 85f007b7-540e-0410-9357-904b9bb8a0f7
Less partial applications in Vars, as well as better memory allocation.

git-svn-id: svn+ssh://scm.gforge.inria.fr/svn/coq/trunk@17065 85f007b7-540e-0410-9357-904b9bb8a0f7