Support variants with large boards (up to 12x10)

[ianfab]

The board size is currently limited to be smaller than or equal to 8x8 squares. Investigate feasibility of switchover to larger bitboards, e.g., based on compiler-specific 128 bit integers, in order to be able to support board sizes up to 12x10.

Required steps:

• [x] use unsigned __int128 for bitboards
• [x] rewrite bitboard operations to work for 128bit (including popcount, pext, etc.)
• [x] extend squares, files, ranks, move representation, etc. to match a 12x10 board
• [x] adjust code for magics for 12x10 / 128 bit bitboards
• [x] fix potential issues with two digit numbers in FEN and move strings
• [x] extend TT entry to use 32 bit int for move representation to avoid wrong TT moves
• [x] generalize castling for large boards
• [x] improve performance of non-pext magics generation (precompute magics)
• [x] make parsing of en passant flag in FEN work irrespective of board size

Further improvements:

• [x] Support MSVC
• [x] Support 32-bit hardware
• [x] reduce size of rook magics for large-board version (~200 MB) by splitting into horizontal and vertical slider
• [x] reduce memory consumption of continuation history table (100 MB) (e.g., by only using slots for piece types actually used in a variant)
• [x] fix endgame code (pawn bitbases, square normalization, etc.) and re-enable it
• [x] fix or disable TB code
• [x] fix issues in evaluation related to different board size (e.g., PSQT asymmetry)
• [x] rewrite move encoding, simplify from_sq

[ianfab]

The implementation for shogi and capablanca chess seems to be working now (tested against Shokidoki and Sjaak II). A couple of workarounds should still be fixed though, but they only affect playing strength and code maintainability. I am not aware of any play-limiting bug.

[ianfab]

Added support for four more large-board variants on branch large_boards:

• Janus chess
• Embassy chess
• Jeson Mor
• Courier chess

[ianfab]

Merged large_boards to master. Keeping this open to track the open issues with the implementation, see the initial post.