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

applications/test/sizeof/Test-sizeof.C

@@ -33,7 +33,7 @@ Description
 #include "Switch.H"
 #include "string.H"
 #include "dictionary.H"
-#include "nil.H"
+#include "zero.H"
 #include "IOstreams.H"
 #include "PstreamBuffers.H"
 #include "argList.H"
@@ -82,8 +82,8 @@ int main(int argc, char *argv[])
     }
 
     {
-        nil x;
-        cout<<"nil:" << sizeof(x) << nl;
+        zero x;
+        cout<<"zero:" << sizeof(x) << nl;
     }
     #if 0
     {