Reintroduce operator>>() in Book class

No functional change.

Signed-off-by: Marco Costalba <mcostalba@gmail.com>

src/book.h

@@ -48,7 +48,7 @@ public:
   const std::string name() const { return bookName; }
 
 private:
-  template<typename T> void get_number(T& n);
+  template<typename T> Book& operator>>(T& n);
 
   BookEntry read_entry(int idx);
   int find_entry(uint64_t key);