add pointer distance function

ranges/ptr_arith.nim

@@ -3,5 +3,8 @@ proc baseAddr*[T](x: openarray[T]): pointer = cast[pointer](x)
 proc shift*(p: pointer, delta: int): pointer {.inline.} =
   cast[pointer](cast[int](p) + delta)
 
+proc distance*(a, b: pointer): int {.inline.} =
+  cast[int](b) - cast[int](a)
+
 proc shift*[T](p: ptr T, delta: int): ptr T {.inline.} =
   cast[ptr T](shift(cast[pointer](p), delta * sizeof(T)))