srfi-18: Use srfi-35 conditions.

* module/srfi/srfi-18.scm: Use srfi-35 conditions instead of our
  home-grown equivalent system.
  (thread-exception-handlers): Remove unused table.
  (srfi-18-exception-handler): Always capture key consed to args; no
  special case for bare key.
* test-suite/tests/srfi-18.test (provided?): Adapt to reason always
  being key+args.

test-suite/tests/srfi-18.test

@@ -484,7 +484,7 @@
         (with-exception-handler
          (lambda (obj)
            (and (uncaught-exception? obj)
-                (eq? (uncaught-exception-reason obj) 'foo)
+                (equal? (uncaught-exception-reason obj) '(foo))
                 (set! success #t)))
          (lambda () (thread-join! t)))
         success)))))