Change the representation of exception slots: instead of being represented as 'string ref', they are now blocks
    of size 2, with tag = Object_tag, the first field being the pointer to the string, and second one being a unique id, generated
    from the same sequence as for object values.  Special case for predefined exceptions, represented with a negative id.
    
    The unique id generator is moved from camlinternalOO to the C runtime system.
    
    Also fix some bugs.
    
    
    
    git-svn-id: http://caml.inria.fr/svn/ocaml/branches/raise_variants@14239 f963ae5c-01c2-4b8c-9fe0-0dff7051ff02