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-unknown.scm

@@ -0,0 +1,17 @@
+(use-modules (knots)
+             (knots backtraces)
+             (fibers)
+             (system repl debug))
+
+(start-stack
+ #t
+ (with-exception-handler
+     (lambda (exn)
+       (let* ((stack (make-stack #t))
+              (stack-classification
+               (classify-stack-situation (stack->vector stack))))
+         (print-backtrace-and-exception/knots exn)
+         (simple-format/knots #t "situation: ~A\n" stack-classification)
+         (primitive-exit 0)))
+   (lambda ()
+     (error "test"))))