ENH: relocate zero_one to pTraits, allows reuse for scalar clamping

ENH: add pTraits and IO for std::int8_t

STYLE: cull some implicitly available includes

- pTraits.H is included by label/scalar etc
- zero.H is included by UList

STYLE: cull redundant forward declarations for Istream/Ostream

src/OpenFOAM/primitives/functions/Function1/ramp/ramp.H

@@ -100,7 +100,7 @@ protected:
         //- that forms the basis of many more complex ramp functions
         inline scalar linearRamp(const scalar t) const
         {
-            return clamp((t - start_)/duration_, 0, 1);
+            return clamp((t - start_)/duration_, zero_one{});
         }