ftw: Add `scandir'.

Suggested by Nala Ginrut <nalaginrut@gmail.com>.

* module/ice-9/ftw.scm (scandir): New procedure.
* test-suite/tests/ftw.test ("scandir"): New test prefix.
* doc/ref/misc-modules.texi (File Tree Walk): Document `scandir'.

test-suite/tests/ftw.test

@@ -166,3 +166,11 @@
                        expected)))
              (_ #f))
             children)))))
+
+(with-test-prefix "scandir"
+
+  (pass-if "test-suite"
+    (let ((select? (cut string-suffix? ".test" <>)))
+      (match (scandir (string-append %test-dir "/tests") select?)
+        (("." ".." "00-initial-env.test" (? select?) ...)
+         #t)))))