clojure.data.xml.preload: preloadable reader tags

This can be required to make xml reader tags work, but loading the
impl nss only when used.

This makes AOT-ing projects that use #xml/ns in the (ns clause)
possible, by requiring clojure.data.xml.preload from user.clj

src/main/clojure/clojure/data/xml.clj

@@ -14,6 +14,7 @@
 
   (:require
    (clojure.data.xml
+    [preload]
     [process :as process]
     [impl :refer [export-api]]
     [node :as node]

src/main/clojure/clojure/data/xml/preload.clj

@@ -0,0 +1,9 @@
+(ns clojure.data.xml.preload)
+
+(defn xml-ns [uri]
+  (require 'clojure.data.xml.name)
+  (clojure.data.xml.name/uri-symbol uri))
+
+(defn xml-element [el]
+  (require 'clojure.data.xml.node)
+  (clojure.data.xml.node/tagged-element el))

src/main/clojure/data_readers.cljc

@@ -1,2 +1,2 @@
-{xml/ns clojure.data.xml.name/uri-symbol
- xml/element clojure.data.xml.node/tagged-element}
+{xml/ns clojure.data.xml.preload/xml-ns
+ xml/element clojure.data.xml.preload/xml-element}