read-print: Correctly support multiple same-named newline forms.

Previously (home-environment (services ...)) would not be considered a
"newline form".  This fixes it.

* guix/read-print.scm (newline-form?): Use 'vhash-foldq*' instead of
'vhash-assq' and iterate over candidates.
* tests/read-print.scm: Add test.

guix/read-print.scm

@@ -367,10 +367,9 @@ surrounding SYMBOL."
 (define (newline-form? symbol context)
   "Return true if parenthesized expressions starting with SYMBOL must be
 followed by a newline."
-  (match (vhash-assq symbol %newline-forms)
-    (#f #f)
-    ((_ . prefix)
-     (prefix? prefix context))))
+  (let ((matches (vhash-foldq* cons '() symbol %newline-forms)))
+    (find (cut prefix? <> context)
+          matches)))
 
 (define (escaped-string str)
   "Return STR with backslashes and double quotes escaped.  Everything else, in

tests/read-print.scm

@@ -294,6 +294,11 @@ mnopqrstuvwxyz.\")"
   ;; page break above
   end)")
 
+(test-pretty-print "\
+(home-environment
+  (services
+   (list (service-type home-bash-service-type))))")
+
 (test-pretty-print/sequence "\
 ;;; This is a top-level comment.