Make sure folly::dynamic arrays can be used with folly::gen

Summary:
Define a value_type for folly::dynamic to make arrays work with
folly::gen::from.

Test Plan:
- added new unit test
- fbconfig folly/test && fbmake runtests

Reviewed By: tjackson@fb.com

FB internal diff: D1216358

folly/dynamic.h

@@ -110,6 +110,7 @@ private:
   typedef std::vector<dynamic> Array;
 public:
   typedef Array::const_iterator const_iterator;
+  typedef dynamic value_type;
   struct const_key_iterator;
   struct const_value_iterator;
   struct const_item_iterator;

folly/test/DynamicTest.cpp

@@ -20,6 +20,7 @@
 
 #include "folly/Benchmark.h"
 #include "folly/dynamic.h"
+#include "folly/gen/Base.h"
 #include "folly/json.h"
 
 using folly::dynamic;
@@ -305,6 +306,13 @@ TEST(Dynamic, GetPtr) {
   EXPECT_EQ(dynamic(2), *cobject.get_ptr("two"));
 }
 
+TEST(Dynamic, ArrayGenerator) {
+  // Make sure arrays can be used with folly::gen.
+  using namespace folly::gen;
+  dynamic arr { 1, 2, 3, 4 };
+  EXPECT_EQ(from(arr) | take(3) | member(&dynamic::asInt) | sum, 6);
+}
+
 int main(int argc, char** argv) {
   testing::InitGoogleTest(&argc, argv);
   google::ParseCommandLineFlags(&argc, &argv, true);