Add generation of diagonal masks

These still need to be tested, though.
Introduce Pradyumna Kannan's magic bitboard generator

Not as if I'm going to write one that's as good as his.
Switch over to new magic bitboard generation

Get rid of my mask generators, and add a call to initmagicmoves().
Add FILES and RANKS arrays in utils

They're self-descriptive. Also, add a call to init_utils() in the
Init_onyx function.