read-print: Define forms for which \n, \t, etc. are not escaped.

read-print: Define forms for which \n, \t, etc. are not escaped.

Previously, the pretty-printer would unconditionally leave everything
but double-quotes and backslashes unescaped when rendering a string.
With this change, the previous behavior only applies to forms listed in
%NATURAL-WHITESPACE-STRING-FORMS.

* guix/read-print.scm (%natural-whitespace-string-forms): New variable.
(printed-string): New procedure.
(pretty-print-with-comments): Use it instead of 'escaped-string'.
* tests/read-print.scm: Add test.