* tests/value_interop.cc (int_u<n>): Augment.
	Deactivate some parts of code because they contain too few
	tests to be relevant; they have to be augmented!
	* tests/value_int_u8.cc: Augment.
	* tests/hexa.cc: Fix; make it compile.
	* mln/core/ops.hh (++, --): New operator default impls.
	* mln/core/concept/value.hh (++, --): Move to...
	* mln/value/concept/scalar.hh: ...here.
	Those ops are only relevant for scalars.
	* mln/value/int_s.hh,
	* mln/value/int_u.hh (+=, -=): Move to...
	* mln/value/concept/integer.hh: ...here.
	Their definition is more general than just for int_*.


git-svn-id: https://svn.lrde.epita.fr/svn/oln/trunk@1275 4aad255d-cdde-0310-9447-f3009e2ae8c0