Remove default soft stack limit; add call-with-stack-overflow-handler

Remove default soft stack limit; add call-with-stack-overflow-handler

* libguile/vm.h:
* libguile/vm.c (default_max_stack_size, initialize_default_stack_size):
  Remove the default stack limit.  In this way, programs run from the
  command line or outside of the REPL will have no soft stack limit.
  (make_vm): Change `max_stack_size' field to be a stack of limits and
  handlers.
  (current_overflow_size, should_handle_stack_overflow)
  (reset_stack_limit, wind_overflow_handler, unwind_overflow_handler)
  (vm_expand_stack): If the stack surpasses a user-set limit, call the
  user-specified handler within its outer stack limit.
  (call-with-stack-overflow-handler): New interface.

* module/system/vm/vm.scm: Export call-with-stack-overflow-handler.