Add cljs.core/rsubseq

Copy & paste from Clojure's core.clj, with minor adjustments.

src/cljs/cljs/core.cljs

@@ -5295,6 +5295,21 @@ reduces them without incurring seq initialization"
        (take-while (mk-bound-fn sc end-test end-key)
                    (if ((mk-bound-fn sc start-test start-key) e) s (next s))))))
 
+(defn rsubseq
+  "sc must be a sorted collection, test(s) one of <, <=, > or
+  >=. Returns a reverse seq of those entries with keys ek for
+  which (test (.. sc comparator (compare ek key)) 0) is true"
+  ([sc test key]
+     (let [include (mk-bound-fn sc test key)]
+       (if (#{< <=} test)
+         (when-let [[e :as s] (-sorted-seq-from sc key false)]
+           (if (include e) s (next s)))
+         (take-while include (-sorted-seq sc false)))))
+  ([sc start-test start-key end-test end-key]
+     (when-let [[e :as s] (-sorted-seq-from sc end-key false)]
+       (take-while (mk-bound-fn sc start-test start-key)
+                   (if ((mk-bound-fn sc end-test end-key) e) s (next s))))))
+
 (deftype Range [meta start end step ^:mutable __hash]
   Object
   (toString [this]