Fix core:slice.rotate_left

This commit includes two fixes:
- a temporary cast to make the function compile
- a fix to a logic error that caused the function to hang or return
  incorrect results

core/slice/ptr.odin

@@ -73,25 +73,17 @@ ptr_rotate :: proc(left: int, mid: ^$T, right: int) {
 		left, mid, right := left, mid, right
 
 		// TODO(bill): Optimization with a buffer for smaller ranges
-		if left >= right {
-			for {
-				ptr_swap_non_overlapping(ptr_sub(mid, right), mid, right)
+		for left > 0 && right > 0 {
+			if left >= right {
+				ptr_swap_non_overlapping(ptr_sub(mid, right), mid, right * size_of(T))
 				mid = ptr_sub(mid, right)
 
 				left -= right
-				if left < right {
-					break
-				}
-			}
-		} else {
-			for {
-				ptr_swap_non_overlapping(ptr_sub(mid, left), mid, left)
+			} else {
+				ptr_swap_non_overlapping(ptr_sub(mid, left), mid, left * size_of(T))
 				mid = ptr_add(mid, left)
 
 				right -= left
-				if right < left {
-					break
-				}
 			}
 		}
 	}