Use more Natural and Positive numbers in proc parameters

- Didn't go through all modules, only the main ones I thought of
- Building the compiler and tests still work

lib/pure/algorithm.nim

@@ -24,7 +24,7 @@ proc `*`*(x: int, order: SortOrder): int {.inline.} =
   var y = order.ord - 1
   result = (x xor y) - y
 
-proc reverse*[T](a: var openArray[T], first, last: int) =
+proc reverse*[T](a: var openArray[T], first, last: Natural) =
   ## reverses the array ``a[first..last]``.
   var x = first
   var y = last
@@ -37,7 +37,7 @@ proc reverse*[T](a: var openArray[T]) =
   ## reverses the array `a`.
   reverse(a, 0, a.high)
 
-proc reversed*[T](a: openArray[T], first, last: int): seq[T] =
+proc reversed*[T](a: openArray[T], first, last: Natural): seq[T] =
   ## returns the reverse of the array `a[first..last]`.
   result = newSeq[T](last - first + 1)
   var x = first