Revise xposts internal representation

[n-osborne]

In the current version, exceptional post-conditions are represented with the type : (xsymbol * (pattern * term) list) list

This doesn't work well with exception that doesn't have any arguments. For example in:

exception E

val f : int -> int
(*@ y = f x
    raises E -> x = 42

the type checker has to fill for the pattern.
It wil linsert a tuple0 pattern, I believe because the type of E is Exn_tuple [].

[n-osborne]

After further investigation, the tuple0 is placed by the parser.
In case we have a pattern but no term (raises E _), the parser will insert a Ttrue.

I believe there are two ways to go:

1. either flatten the type and go for somethin along the way of (xsymbol * pattern option * term) list
2. add pattern for exceptions and go for (pattern * term) list which is even simpler