Revert "Fast lookup_named in environments, based on maps instead of lists."

Contrarily to my machine results, it seems that it tore down the performance of
Coq on benchmarks.

git-svn-id: svn+ssh://scm.gforge.inria.fr/svn/coq/trunk@17091 85f007b7-540e-0410-9357-904b9bb8a0f7