Better implementation for stores.

git-svn-id: svn+ssh://scm.gforge.inria.fr/svn/coq/trunk@17093 85f007b7-540e-0410-9357-904b9bb8a0f7
Slightly faster version of merging substitutions in TacticMatching.

git-svn-id: svn+ssh://scm.gforge.inria.fr/svn/coq/trunk@17094 85f007b7-540e-0410-9357-904b9bb8a0f7