#6203: change representation of exception values created with a constant constructor: the value is now equal to the exception slot.  This avoids some allocation when the constructor is called and an extra indirection on matching against the constructor.

git-svn-id: http://caml.inria.fr/svn/ocaml/branches/raise_variants@14235 f963ae5c-01c2-4b8c-9fe0-0dff7051ff02