factor out post-link method

lib/Render/Util.rakumod

@@ -1,6 +1,8 @@
 use v6.e.PREVIEW;
 unit module Render::Util;
 
+use DB::Post;
+
 sub opt($test, $item) is export {
     if $test {
         $item
@@ -16,3 +18,14 @@ sub optl($test, &item) is export {
         []
     }
 }
+
+#| Link to the post by the primary slug, if there is one, linking to it by id
+#| otherwise
+sub post-link(Int:D $id, Post:D $post --> Str:D) is export {
+    my @slugs = $post.all-slugs;
+    if @slugs {
+       "/posts/by-slug/{@slugs[*-1]}.html"
+    } else {
+       "/posts/by-id/$id.html"
+    }
+}