Add lightweight keyset wrapper

Saves on allocation and provides an immutable facade to underlying
regular set

src/malli/core.cljc

@@ -337,6 +337,35 @@
      :cljs
      (.slice arr 0 to)))
 
+#?(:clj
+   (defn- -set-wrapper
+     [^java.util.Set s]
+     (reify
+       Collection
+       (size [_] (.size s))
+       (containsAll [_ c] (.containsAll s c))
+       clojure.lang.IPersistentSet
+       (disjoin [_ key] (disj (set s) key))
+       (contains [_ key] (.contains s key))
+       (get [_ key] (when (.contains s key) key))
+       clojure.lang.Counted
+       (count [_] (.size s))
+       clojure.lang.IPersistentCollection
+       (cons [_ o] (.cons (set s) o))
+       (empty [_] #{})
+       (equiv [_ o]
+         (if (instance? java.util.Set o)
+           (if (== (.size s) (.size ^Collection o))
+             (.containsAll s o)
+             false)
+           false))
+       clojure.lang.Seqable
+       (seq [_] (seq s))
+       clojure.lang.IFn
+       (invoke [_ o] (when (.contains s o) o))
+       Iterable
+       (iterator [_] (.iterator s)))))
+
 (defn -parse-entries
   [children {:keys [naked-keys lazy-refs]} options]
   (let [n (count children)
@@ -351,11 +380,11 @@
           {:children (arr->vec -children)
            :entries (arr->vec -entries)
            :forms (arr->vec -forms)
-           :keyset (#?(:clj set, :cljs clojure.core/deref) -keyset)}
+           :keyset (#?(:clj -set-wrapper, :cljs clojure.core/deref) -keyset)}
           {:children (arr->vec (arange -children i))
            :entries (arr->vec (arange -entries i))
            :forms (arr->vec (arange -forms i))
-           :keyset (#?(:clj set, :cljs clojure.core/deref) -keyset)})
+           :keyset (#?(:clj -set-wrapper, :cljs clojure.core/deref) -keyset)})
         (recur
          (-parse-entry* (nth children i) naked-keys lazy-refs options i -children -entries -forms -keyset)
          (unchecked-inc-int ci))))))