Support approximateSize() on db bigger than 4 GB (#175)

By using napi_create_int64 instead of napi_create_int32.

Ported from Level/leveldown#777.

Author: larskuhtz

binding.cc

@@ -1077,7 +1077,7 @@ struct ApproximateSizeWorker final : public PriorityWorker {
   void HandleOKCallback () override {
     napi_value argv[2];
     napi_get_null(env_, &argv[0]);
-    napi_create_uint32(env_, (uint32_t)size_, &argv[1]);
+    napi_create_int64(env_, (int64_t)size_, &argv[1]);
     napi_value callback;
     napi_get_reference_value(env_, callbackRef_, &callback);
     CallFunction(env_, callback, 2, argv);