tags: doc tidy.
traits: added remove_cv<T>.
traits: added integral_constant<T, V>.
traits: added type_if<C, T, E>.
traits: made false_type & true_type alias integral_constant<T, V>.
traits: don't remove reference in is_signed<T> & is_unsigned<T>.
traits: tidy; doc tidy.
traits: added is_floating_point<T>; moved is_same<T, U> above type categories.
traits: added is_pointer<T>, is_reference<T>, & is_floating_point<T>.
traits: removed bool from is_integral<T> consideration; doc tidy.
traits: renamed integral_constant to constant_type.
debug_constraints: removed use of stdlib where possible.¹

1: Due to difference of definition in is_integral<T> and
is_arithmetic<T> (due to is_integral<T>), this changes the
constraints slightly. None of it affects the intentions of
constraints already in use in togo.
debug_constraints: removed TOGO_CONSTRAIN_INTEGRAL_ARITHMETIC(T).¹

1: By stdlib definition, this was effectively only doing
is_integral<T>.
utility: constrain only on integrals in min(), max(), & clamp().¹

1: With the now-removed "comparable" constraint, it *was* possible
to call these functions with a pointer.
Replaced "integral arithmetic" constraints with integral constraints.
debug_constraints: added to debug doc-group.
doc: tidy.