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.

tests/read-print.scm

@@ -186,6 +186,9 @@ expressions."
               (lambda _
                 xyz))))")
 
+(test-pretty-print "\
+(string-append \"a\\tb\" \"\\n\")")
+
 (test-pretty-print "\
 (description \"abcdefghijkl
 mnopqrstuvwxyz.\")"