Runtime: Added flags field for objects.
Runtime: Added flag to clear objects on free.

The PROGRAM_CLEAR_STORAGE flag now sets the new object flag
OBJECT_CLEAR_ON_EXIT (analogous to STRING_CLEAR_ON_EXIT) at
clone time.
Object: Added secure().

This function sets the OBJECT_CLEAR_ON_EXIT flag on the provided
object (analogous to String.secure() and STRING_CLEAR_ON_EXIT).

Also fixes the documented type of String.secure().