read-print: 'read-with-comments' reads comments within gexps.

Fixes <https://issues.guix.gnu.org/62059>.
Reported by Maxim Cournoyer <maxim.cournoyer@gmail.com>.

* guix/read-print.scm (read-with-comments): Special-case #~, #$, and #+.
* tests/read-print.scm: Add two tests.

tests/read-print.scm

@@ -231,6 +231,21 @@ mnopqrstuvwxyz.\")"
   ;; Regular indentation for 'replace' here.
   (replace \"gmp\" gmp))")
 
+(test-pretty-print "\
+#~(modify-phases phases
+    (add-after 'whatever 'something-else
+      (lambda _
+        ;; This comment appears inside a gexp.
+        42)))")
+
+(test-pretty-print "\
+#~(list #$@(list coreutils ;yup
+                 grep) ;margin comment
+        #+sed
+
+        ;; Line comment.
+        #$grep)")
+
 (test-pretty-print "\
 (package
   ;; Here 'sha256', 'base32', and 'arguments' must be