Fast lookup_named in environments, based on maps instead of lists.
This was quite a severe performance bottleneck.

Ideally, this data structure should be put into contexts, but the relevant
type is transparent... For now, we stick to this inelegant workaround.

git-svn-id: svn+ssh://scm.gforge.inria.fr/svn/coq/trunk@17086 85f007b7-540e-0410-9357-904b9bb8a0f7