Micro-optimize castleRights update

When updating castleRights in do_move() perform only one
64bit xor with zobCastle[] instead of two.

The trick here is to define zobCastle[] keys of composite
castling rights as a xor combination of the keys of the
single castling rights, instead of 16 independent keys.

Idea from Critter although implementation is different.

No functional change.

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