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/vhash-fold.scm

@@ -0,0 +1,24 @@
+(use-modules (knots) (ice-9 vlist))
+
+;; LAST BACKTRACE ENTRY: 257:2
+
+(define (do-fold)
+  (begin
+    (vhash-fold
+     (lambda (key value result)
+       ;; Shouldn't be reached
+       #f)
+     0
+     ;; The aim here is to pass in #f for the vlist, and cause an
+     ;; exception within the (ice-9 vlist) module
+     #f)
+    'done))
+
+;; FIRST BACKTRACE ENTRY: 1762:12    (with-exception-handler
+(with-exception-handler
+    (lambda (exn)
+      (print-backtrace-and-exception/knots exn)
+      (primitive-exit 1))
+  (lambda ()
+    (do-fold)
+    'done))