Add support for large-board variants (#6)

Add support for large-board variants (#6)

Add compile-time option for enabling a 12x10 board representation with
128 bit bitboards to enable support for chess variants with large boards.
Support shogi as first variant with board size >8x8.

Open issues for large board version:
- Evaluation needs to be consolidated (esp. PSQT)
- Endgame evaluation is disabled and requires to be fixed
  before reenabling it.
- Memory consumption of history tables needs to be reduced.

No functional change for normal version.