Fix exn dispatch for exns within pre-unwind handlers

* libguile/exceptions.c (exception_epoch_fluid): Rename from
active_exception_handlers_fluid.
(scm_dynwind_throw_handler): Increment exception epoch instead of
resetting active exception handlers.
(scm_init_exceptions): Update.
* module/ice-9/boot-9.scm  (with-exception-handler): Rework to associate
an "epoch" fluid with each exception handler.
(with-throw-handler): Establish a new epoch, during the execution of a
throw handler.
(raise-exception): Rework to avoid capturing a list of exception
handlers, and to use epochs as a way to know which handlers have already
been examined and which are on the dispatch stack.
* test-suite/tests/exceptions.test ("throwing within exception
handlers"): New test.

test-suite/tests/exceptions.test

@@ -17,6 +17,7 @@
 
 
 (define-module (test-suite exceptions)
+  #:use-module (ice-9 control)
   #:use-module (test-suite lib))
 
 (define-syntax-parameter push
@@ -392,3 +393,14 @@
       (let* ((thunk1 (catch* 'foo (lambda () (throw 'bar))))
              (thunk2 (catch* 'bar (lambda () (thunk1)))))
         (thunk2))))
+
+(with-test-prefix "throwing within exception handlers"
+  (pass-if "https://github.com/wingo/fibers/issues/76"
+    (let/ec return
+      (with-exception-handler
+          (lambda (e)
+            (catch #t
+              (lambda () (error "bar"))
+              (lambda args #f))
+            (return #t))
+        (lambda () (error "foo"))))))