Evarconv: When doing a iota of a fixpoint, use constant name instead of fixpoint definition

+ Help the use of #trace on evar_conv_appr_x

git-svn-id: svn+ssh://scm.gforge.inria.fr/svn/coq/trunk@16244 85f007b7-540e-0410-9357-904b9bb8a0f7
cbn friendly VectorDef

git-svn-id: svn+ssh://scm.gforge.inria.fr/svn/coq/trunk@16245 85f007b7-540e-0410-9357-904b9bb8a0f7