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/blenkernel/intern/mesh_validate.c

@@ -1486,7 +1486,7 @@ void BKE_mesh_calc_edges(Mesh *mesh, bool update, const bool select)
 		}
 	}
 
-	totedge = BLI_edgehash_size(eh);
+	totedge = BLI_edgehash_len(eh);
 
 	/* write new edges into a temporary CustomData */
 	CustomData_reset(&edata);