Cleaning up of Evd. Extruding the tower of modules used to define evar_maps.
They did not really enhance the safety of the code, as several outer parts
acceeded directly to the internal representation.

git-svn-id: svn+ssh://scm.gforge.inria.fr/svn/coq/trunk@16762 85f007b7-540e-0410-9357-904b9bb8a0f7
Documentation of Evd.

git-svn-id: svn+ssh://scm.gforge.inria.fr/svn/coq/trunk@16763 85f007b7-540e-0410-9357-904b9bb8a0f7