0.9.5.83: almost fix COMPUTE-RESTARTS & FIND-RESTART
* back out the previous change to COMPUTE-RESTARTS.
* make FIND-RESTART check the activity, not applicability, of a restart object
passed in as an argument.
=> now both the Helmut Eller's case and ansi-tests pass.
Although: I dispute COMPUTE-RESTARTS.10 and RESTART-BIND.20. See emails to
sbcl-devel around Sun, 16 Oct 2005 13:12. The more radical changes outlined there
are not part of this yet.