let SharedMutexToken export a nice interface

Summary: Basically it needs a nice constructor and `operator bool`.

Reviewed By: iahs

Differential Revision: D31188190

fbshipit-source-id: ec64d944674b0879e045aa92df95c592f66c3be4

folly/SharedMutex.h

@@ -247,8 +247,12 @@ struct SharedMutexToken {
     DEFERRED_SHARED,
   };
 
-  Type type_;
-  uint16_t slot_;
+  Type type_{};
+  uint16_t slot_{};
+
+  constexpr SharedMutexToken() = default;
+
+  explicit operator bool() const { return type_ != Type::INVALID; }
 };
 
 #ifndef FOLLY_SHARED_MUTEX_MAX_SPIN_DEFAULT