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-map.scm

@@ -0,0 +1,20 @@
+(use-modules (knots) (fibers) (knots parallelism))
+
+(run-fibers
+ (lambda ()
+   (with-exception-handler
+       (lambda _
+         ;; To avoid the test hanging if there's an exception
+         (primitive-exit 1))
+     (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-map
+            (lambda (x)
+              (error "error from fibers-map"))  ; LAST BACKTRACE ENTRY HERE
+            '(1)))))))
+ #:hz 0 #:parallelism 1)