Have fun with union in book.cpp

Fancy way to use an union to map polyglot
zobrist keys in one go.

Also some renaming while there.

No functional change.

src/book.h

@@ -36,9 +36,9 @@ private:
   template<typename T> PolyglotBook& operator>>(T& n);
 
   bool open(const char* fName);
-  size_t find_first(uint64_t key);
+  size_t find_first(Key key);
 
-  RKISS RKiss;
+  RKISS rkiss;
   std::string fileName;
 };