Minimal implementation of `Shallow marshalling of Lib

Just sufficient to make abstract work in such an (empty) lib.

git-svn-id: svn+ssh://scm.gforge.inria.fr/svn/coq/trunk@16772 85f007b7-540e-0410-9357-904b9bb8a0f7
CoqIDE: show number of proofs being checked in background

good test: Nijmegen/QArithSternBrocot/Zaux.v

git-svn-id: svn+ssh://scm.gforge.inria.fr/svn/coq/trunk@16773 85f007b7-540e-0410-9357-904b9bb8a0f7