* stacks.c: Improve selection of relevant stack frames when making

* stacks.c: Improve selection of relevant stack frames when making
a stack object.  Introduce one level of indirection in the stack
object to make it possible to "narrow" to a certain region of the
stack.  This facilitates making use of more clever algorithms (not
implemented) for selecting relevant frames and gives a cleaner
design since selection of frames can be done independently of
extraction of frames from the real stack.
(scm_stack_id): Also take #t as argument which means look at
current stack.