[Ival] Implements fold_int_decreasing, which iterates on decreasing order.

src/kernel_services/abstract_interp/int_interval.ml

src/kernel_services/abstract_interp/int_interval.mli

src/kernel_services/abstract_interp/int_set.ml

src/kernel_services/abstract_interp/int_set.mli

src/kernel_services/abstract_interp/int_val.ml

src/kernel_services/abstract_interp/int_val.mli

src/kernel_services/abstract_interp/ival.ml

src/kernel_services/abstract_interp/ival.mli