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-resource-from-pool.scm

@@ -0,0 +1,16 @@
+(use-modules (knots) (fibers) (knots resource-pool))
+
+(run-fibers
+ (lambda ()
+   (let ((pool (make-resource-pool (const 'resource) 1)))
+     ;; 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-resource-from-pool pool
+                                       (lambda (resource)
+                                         (error "error from call-with-resource-from-pool")))))))  ; LAST BACKTRACE ENTRY HERE
+ #:hz 0 #:parallelism 1)
+