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.

Makefile.am

@@ -2,18 +2,20 @@ include guile.am
 
 SOURCES =			\
   knots.scm			\
+  knots/backtraces.scm		\
   knots/non-blocking.scm	\
   knots/parallelism.scm	\
   knots/promise.scm		\
-  knots/web.scm		\
   knots/queue.scm		\
   knots/resource-pool.scm	\
   knots/sort.scm		\
   knots/thread-pool.scm	\
   knots/timeout.scm		\
-  knots/web-server.scm
+  knots/web-server.scm		\
+  knots/web.scm
 
 SCM_TESTS =			\
+  tests/backtraces.scm		\
   tests/non-blocking.scm	\
   tests/non-blocking.scm	\
   tests/parallelism.scm	\