Avoids a compilation warning with ocaml >= 4.08.

Avoids a compilation warning with ocaml >= 4.08.

Avoids a compilation warning with ocaml >= 4.08.

Shares the implementation of the checks between the two files.

In int_val, removes specific code handling empty sets.
Modifies the function [diff_if_one] that created a set with too many elements
before removing one element.

Fixes #613.

Int_val uses the limit provided by int_set.get_small_cardinal to create
sets or intervals.

In Ival, bottom is now a special constructor.