CLJS-491: avoid layered subvecs

cljs.core/build-subvec now checks whether its vector argument is
already a Subvec and, if so, unwraps it, so that the return value of
cljs.core/subvec is always a view on a regular vector. This is in line
with the behaviour of subvec in Clojure.

Includes a basic test.

src/cljs/cljs/core.cljs

@@ -3359,13 +3359,15 @@ reduces them without incurring seq initialization"
     (-lookup coll k not-found)))
 
 (defn- build-subvec [meta v start end __hash]
-  (let [c (count v)]
-       (when (or (neg? start)
-                 (neg? end)
-                 (> start c)
-                 (> end c))
-         (throw (js/Error. "Index out of bounds")))
-       (Subvec. meta v start end __hash)))
+  (if (instance? Subvec v)
+    (recur meta (.-v v) (+ (.-start v) start) (+ (.-start v) end) __hash)
+    (let [c (count v)]
+      (when (or (neg? start)
+                (neg? end)
+                (> start c)
+                (> end c))
+        (throw (js/Error. "Index out of bounds")))
+      (Subvec. meta v start end __hash))))
 
 (defn subvec
   "Returns a persistent vector of the items in vector from

test/cljs/cljs/core_test.cljs

@@ -1047,6 +1047,8 @@
     (assert (= :fail (try (subvec v2 6 10) (catch js/Error e :fail))))
     (assert (= :fail (try (subvec v2 6 10) (catch js/Error e :fail))))
     (assert (= :fail (try (subvec v2 3 6) (catch js/Error e :fail))))
+    ;; no layered subvecs
+    (assert (identical? v1 (.-v (subvec s 1 4))))
     )
 
   ;; TransientVector