CLJS-3290: implement IHash for js Symbol (#225)

* implement IHash for js Symbol
* add test cases

Author: swannodette

src/main/cljs/cljs/core.cljs

@@ -1458,6 +1458,11 @@
   (-hash [o]
     (goog/getUid o)))
 
+(extend-type symbol
+  IHash
+  (-hash [o]
+    (hash (.toString o))))
+
 ;;this is primitive because & emits call to array-seq
 (defn inc
   "Returns a number one greater than num."

src/test/cljs/cljs/hashing_test.cljs

@@ -102,3 +102,12 @@
     (is (= (hash 0.5) (hash 0.5)))
     (is (= (hash -0.32553251) (hash -0.32553251)))
     (is (= (hash -0.0000032553251) (hash -0.0000032553251)))))
+
+(deftest test-cljs-3290
+  (testing "JS Symbol hash"
+    (let [s (.for js/Symbol "foo")]
+      (is (number? (hash s)))
+      (is (== (hash s) (hash s)))
+      (is (not (== (hash s) (hash (.for js/Symbol "bar")))))
+      (let [m {s 2}]
+        (is (== 2 (get m s)))))))