change throw mechanism to fit better with cleanup mechanism:

- balls are now stored off line and recovered by Prolog code when everything
is safe.
- if a ball exists, throw uses the *existing* one (be careful not to have one
laying around).
- Jump routine cleans up every cp except ones for setup_call handling, backtracking simply calls setup handlers.

packages/plunit/examples/simple.pl

@@ -1,6 +1,7 @@
-:- module(ex_simple, []).
 :- use_module(library(plunit)).
 
+:- module(ex_simple, []).
+
 :- begin_tests(lists).
 
 test(true) :-