Add series links

lib/DB/Series.rakumod

@@ -13,3 +13,12 @@ has Str:D $.desc is required;
 
 #| The ids of the posts in the series, in series order
 has Int:D @.post-ids is rw = [];
+
+#| Returns true if this series contains the given post id
+method contains-post(Int:D $post-id --> Bool:D) {
+    if $post-id ~~ any @!post-ids {
+        True
+    } else {
+        False
+    }
+}