Cleanup: use '_len' instead of '_size' w/ BLI API

- When returning the number of items in a collection use BLI_*_len()
- Keep _size() for size in bytes.
- Keep _count() for data structures that don't store length
  (hint this isn't a simple getter).

See P611 to apply instead of manually resolving conflicts.

source/blender/editors/curve/editcurve_select.c

@@ -1607,7 +1607,7 @@ static void curve_select_shortest_path_surf(Nurb *nu, int vert_src, int vert_dst
 		int axis, sign;
 		int u, v;
 
-		vert_curr = *((int *)BLI_heap_popmin(heap));
+		vert_curr = *((int *)BLI_heap_pop_min(heap));
 		if (vert_curr == vert_dst) {
 			break;
 		}