Implement SRFI-111 Boxes.

* module/srfi/srfi-111.scm: New file.
* module/Makefile.am (SRFI_SOURCES): Add srfi/srfi-111.scm.
* test-suite/tests/srfi-111.test: New file.
* test-suite/Makefile.am (SCM_TESTS): Add tests/srfi-111.test.
* doc/ref/srfi-modules.texi (SRFI-111): New node.

module/srfi/srfi-111.scm

@@ -0,0 +1,37 @@
+;;; srfi-111.scm -- SRFI 111 Boxes
+
+;;      Copyright (C) 2014 Free Software Foundation, Inc.
+;;
+;; This library is free software; you can redistribute it and/or
+;; modify it under the terms of the GNU Lesser General Public
+;; License as published by the Free Software Foundation; either
+;; version 3 of the License, or (at your option) any later version.
+;;
+;; This library is distributed in the hope that it will be useful,
+;; but WITHOUT ANY WARRANTY; without even the implied warranty of
+;; MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+;; Lesser General Public License for more details.
+;;
+;; You should have received a copy of the GNU Lesser General Public
+;; License along with this library; if not, write to the Free Software
+;; Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA
+
+(define-module (srfi srfi-111)
+  #:use-module (srfi srfi-9)
+  #:use-module (srfi srfi-9 gnu)
+  #:export (box box? unbox set-box!))
+
+(cond-expand-provide (current-module) '(srfi-111))
+
+(define-record-type <box>
+  (box value)
+  box?
+  (value unbox set-box!))
+
+(set-record-type-printer! <box>
+  (lambda (box port)
+    (display "#<box " port)
+    (display (number->string (object-address box) 16) port)
+    (display " value: ")
+    (write (unbox box) port)
+    (display ">" port)))