Rename occupied_squares() to pieces()

Also some microoptimizations, were there from ages
but hidden: the renaming suddendly made them visible!

This is a good example of how better naming lets you write
better code. Naming is really a kind of black art!

No functional change.

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

src/book.cpp

@@ -315,7 +315,7 @@ namespace {
   uint64_t book_key(const Position& pos) {
 
     uint64_t key = 0;
-    Bitboard b = pos.occupied_squares();
+    Bitboard b = pos.pieces();
 
     while (b)
     {