Add spawn-fiber/knots

Which is like spawn-fiber, but uses knots exception handling.

knots.scm

@@ -2,6 +2,7 @@
   #:use-module (srfi srfi-1)
   #:use-module (ice-9 match)
   #:use-module (ice-9 suspendable-ports)
+  #:use-module (fibers)
   #:use-module (fibers conditions)
   #:use-module (system repl debug)
   #:export (call-with-default-io-waiters
@@ -15,7 +16,9 @@
             knots-exception?
             knots-exception-stack
 
-            print-backtrace-and-exception/knots))
+            print-backtrace-and-exception/knots
+
+            spawn-fiber/knots))
 
 (define (call-with-default-io-waiters thunk)
   (parameterize
@@ -149,3 +152,31 @@
                 (close-output-port string-port)
                 str)))))
     (display error-string port)))
+
+(define* (spawn-fiber/knots thunk #:optional scheduler #:key parallel?)
+  (spawn-fiber
+   (lambda ()
+     (with-exception-handler
+         (lambda (exn)
+           (display "Uncaught exception in task:\n"
+                    (current-error-port))
+           (print-backtrace-and-exception/knots exn))
+       (lambda ()
+         (with-exception-handler
+             (lambda (exn)
+               (let ((stack
+                      (match (fluid-ref %stacks)
+                        ((stack-tag . prompt-tag)
+                         (make-stack #t
+                                     0 prompt-tag
+                                     0 (and prompt-tag 1)))
+                        (_
+                         (make-stack #t)))))
+                 (raise-exception
+                  (make-exception
+                   exn
+                   (make-knots-exception stack)))))
+           thunk))
+       #:unwind? #t))
+   scheduler
+   #:parallel? parallel?))