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/call-with-cached-connection.scm

@@ -0,0 +1,18 @@
+(use-modules (knots) (fibers) (knots resource-pool) (knots web))
+
+(run-fibers
+ (lambda ()
+   (let ((cache (make-fixed-size-resource-pool
+                 (list (open-input-string "fake")))))
+     ;; FIRST BACKTRACE ENTRY: 1762:12    (with-exception-handler
+     (with-exception-handler
+         (lambda (e)
+           (print-backtrace-and-exception/knots e)
+           (primitive-exit 1))
+       (lambda ()
+         (call-with-cached-connection cache
+                                      (lambda (port)
+                                        (error "error from call-with-cached-connection"))  ; LAST BACKTRACE ENTRY HERE
+                                      #:close-connection-on-exception? #f)))))
+ #:hz 0 #:parallelism 1)
+