data.zip: add clojure.zip/xml-zip; clojure.xml lives in jolt-lang/xml

clojure.zip was missing xml-zip — a zipper over xml {:tag :content} elements,
which clojure.data.zip and any xml-zipper code needs. Added (runtime, loaded on
require). clojure.data.zip's whole xml suite (9/9) then passes, once XML parsing
is provided: clojure.xml/parse now ships in jolt-lang/xml over its
javax.xml.stream pull parser (committed there).

Listed in docs/libraries.md + the site.

docs/libraries.md

@@ -57,6 +57,9 @@ e.g. the [ring-app example](https://github.com/jolt-lang/examples/tree/main/ring
 * [core.contracts](https://github.com/clojure/core.contracts) — programming by
   contract (`contract`/`with-constraints`/`provide`), over
   [core.unify](https://github.com/clojure/core.unify).
+* [data.zip](https://github.com/clojure/data.zip) — zipper navigation, including
+  `clojure.data.zip.xml`; XML parsing via [jolt-lang/xml](https://github.com/jolt-lang/xml)
+  (which now ships `clojure.xml/parse`).
 * [tick](https://github.com/juxt/tick) — date/time over Jolt's `java.time`;
   `#time/…` literals via `time-literals`.
 * [transit-jolt](https://github.com/jolt-lang/transit-jolt) — Transit (JSON) read/write

stdlib/clojure/zip.clj

@@ -28,6 +28,16 @@
   [root]
   (zipper vector? seq (fn [node children] (with-meta (vec children) (meta node))) root))
 
+(defn xml-zip
+  "Returns a zipper for xml elements (as from clojure.xml/parse), given a root
+  element"
+  [root]
+  (zipper (complement string?)
+          (comp seq :content)
+          (fn [node children]
+            (assoc node :content (and children (apply vector children))))
+          root))
+
 (defn node "Returns the node at loc" [loc] (nth loc 0))
 
 (defn branch? "Returns true if the node at loc is a branch"