d: create alias Position for YYPosition

* data/skeletons/d.m4 (b4_public_types_declare): Here.
* data/skeletons/lalr1.d: Adjust.
* doc/bison.texi: Document it.
* examples/d/calc/calc.y: Use it.
* tests/calc.at: Test it.

examples/d/calc/calc.y

@@ -168,12 +168,12 @@ if (isInputRange!R && is(ElementType!R : dchar))
     }
   }
 
-  YYPosition startPos() const
+  Position startPos() const
   {
     return location.begin;
   }
 
-  YYPosition endPos() const
+  Position endPos() const
   {
     return location.end;
   }