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/fibers-force.scm

@@ -0,0 +1,15 @@
+(use-modules (knots) (fibers) (knots promise))
+
+(run-fibers
+ (lambda ()
+   ;; FIRST BACKTRACE ENTRY: 1762:12    (with-exception-handler
+   (with-exception-handler
+       (lambda (e)
+         (print-backtrace-and-exception/knots e)
+         (primitive-exit 1))
+     (lambda ()
+       (fibers-force
+        (fibers-delay
+         (lambda ()
+           (error "error from fibers-force"))))))) ; LAST BACKTRACE ENTRY HERE
+ #:hz 0 #:parallelism 1)