At least made the evar type opaque! There are still 5 remaining unsafe
casts of ints to evars.

- 2 in Evarutil and Goal which are really needed, even though the Goal
one could (and should) be removed;
- 2 in G_xml and Detyping that are there for completeness sake, but
that might be made anomalies altogether;
- 1 in Newring which is quite dubious at best, and should be fixed.

git-svn-id: svn+ssh://scm.gforge.inria.fr/svn/coq/trunk@16786 85f007b7-540e-0410-9357-904b9bb8a0f7