Remove unnecessary copies in dynamic::hash()

Summary:
`dynamic::hash()` would copy every key-value pair in the object accumulator hash because of two bugs in the code:
1. The lambda took `auto` instead of `auto const&`
2. The hasher was `hash<pair<dynamic, dynamic>>` not `hash<pair<dynamic const, dynamic>>` meaning a conversion was needed.

These bugs together caused 2 copies for each sub-object. Since the copies are recursive, each object got copied 2*depth times.

Reviewed By: yfeldblum

Differential Revision: D16452213

fbshipit-source-id: 64a55e1640abb022c148183646e9f9720fd8482e

folly/dynamic.cpp

@@ -292,12 +292,12 @@ std::size_t dynamic::hash() const {
       // Accumulate using addition instead of using hash_range (as in the ARRAY
       // case), as we need a commutative hash operation since unordered_map's
       // iteration order is unspecified.
-      auto h = std::hash<std::pair<dynamic, dynamic>>{};
+      auto h = std::hash<std::pair<dynamic const, dynamic>>{};
       return std::accumulate(
           items().begin(),
           items().end(),
           size_t{0x0B1EC7},
-          [&](auto acc, auto item) { return acc + h(item); });
+          [&](auto acc, auto const& item) { return acc + h(item); });
     }
     case ARRAY:
       return folly::hash::hash_range(begin(), end());