Some optimizations and simplifications in checkStatus and wouldCheck: use
clear function instead of slower attack functions.
Speed enhancement: introduce getDirectionalAttacks function
and use this where possible to avoid calls to magic bitboard
attack functions. For example this replaces the getXRay
function used in see(). About +3 ELO.

Also: move getMinAttacker function to see module, rename
Bitboard::is_clear to isClear, for consistency.