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/nested-parallelism.scm

@@ -0,0 +1,40 @@
+(use-modules (knots) (fibers) (knots parallelism))
+
+;; Deep call chain within the innermost fiber.  Each function calls the next
+;; via `begin', placing the call in non-tail position so Guile's TCO does not
+;; collapse the frames; all four frames survive and appear in the backtrace.
+(define (deeply-nested x)
+  (error "deeply nested error" x))  ; LAST BACKTRACE ENTRY HERE
+
+(define (three-deep x)
+  (fibers-map deeply-nested (list x)))
+
+(define (two-deep x)
+  (fibers-map three-deep (list x)))
+
+(define (one-deep x)
+  (fibers-map two-deep (list x)))
+
+;; process-batch runs inside one fiber and dispatches the deep call chain into
+;; a nested fiber via a second fibers-map, creating two fiber boundaries.
+(define (process-batch items)
+  (begin
+    (fibers-map one-deep (list items))
+    'unreachable))
+
+(define (run-work)
+  (begin
+    (fibers-map process-batch '(1))
+    'unreachable))
+
+(define result
+  (run-fibers
+   (lambda ()
+     ;; FIRST BACKTRACE ENTRY: 1762:12    (with-exception-handler
+     (with-exception-handler
+         (lambda (e)
+           (print-backtrace-and-exception/knots e)
+           (primitive-exit 1))
+       run-work))
+   #:hz 0 #:parallelism 1))
+