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/guile-error-in-thread.scm

@@ -0,0 +1,11 @@
+(use-modules (knots))
+
+;; FIRST BACKTRACE ENTRY: 1762:12    (with-exception-handler
+(with-exception-handler
+    (lambda (exn)
+      (print-backtrace-and-exception/knots exn)
+      (primitive-exit 1))
+  (lambda ()
+    (call-with-temporary-thread
+     (lambda ()
+       (+ 1 'a)))))           ; LAST BACKTRACE ENTRY HERE