[Ada] Make the functional Maps and Sets unbounded

[Ada] Make the functional Maps and Sets unbounded

Before this patch, the Functional Sets ans Maps were bounded both from
the user and the implementation points of view.  To make them closer to
mathematical Sets ans Maps, this patch removes the bounds from the
contracts. Note that, in practice, they are still bounded by
Count_Type'Last, even if the user is not aware of it anymore.

This patch removed constraints on length of sets and maps from the
preconditions of functions. The function Length and Num_Overlaps now
return a Big_Natural.

gcc/ada/

	* libgnat/a-cofuse.ads, libgnat/a-cofuse.adb,
	libgnat/a-cofuma.ads, libgnat/a-cofuma.adb: Make Length and
	Num_Overlaps return Big_Natural.
	* libgnat/a-cforse.ads, libgnat/a-cforse.adb,
	libgnat/a-cforma.adb, libgnat/a-cfhase.ads,
	libgnat/a-cfhase.adb, libgnat/a-cfhama.adb,
	libgnat/a-cfdlli.adb: Adapt code to handle Big_Integers instead
	of Count_Type.