CLJS-1635: Var type implements IEquiv but not IHash

Extends the Var deftype to implement the IHash protocol, with similar
semantics to its IEquiv implementation: two var's should compare and
hash equal iff their symbol is the same.

Author: cvermilion

src/main/cljs/cljs/core.cljs

@@ -1012,6 +1012,9 @@
     (if (instance? Var other)
       (= (.-sym this) (.-sym other))
       false))
+  IHash
+  (-hash [_]
+    (hash-symbol sym))
   Fn
   IFn
   (-invoke [_]