Rework the backtrace handling

Try and improve the print-backtrace-and-exception/knots output, be
selective about the stack frames which are shown to try and produce
some useful output which reflects user code and hide the
&knots-exceptions.

This commit also introduces a bunch of tests scripts that produce
output from print-backtrace-and-exception/knots, to help test this
code in different situations.

tests/backtraces/stack-situation-fibers.scm

@@ -0,0 +1,19 @@
+(use-modules (knots)
+             (knots backtraces)
+             (fibers)
+             (system repl debug))
+
+(run-fibers
+ (lambda ()
+   (with-exception-handler
+       (lambda (exn)
+         (let ((stack (make-stack #t)))
+           (print-backtrace-and-exception/knots exn)
+           (simple-format/knots #t
+                                "situation: ~A\n"
+                                (classify-stack-situation
+                                 (stack->vector stack))))
+         (primitive-exit 0))
+     (lambda ()
+       (error "test"))))
+ #:hz 0 #:parallelism 1)