Refactor Move representation

[mooskagh]

Main motivation was to have consistent move representation in the Move struct, particularly:

• Castling is always encoded as "king takes rook", doesn't matter whether we are in chess960 mode or not.
• The difference only appears during converting from/to string representation.

Some cascading changes:

• Instead of int row, int col which were very error prone, have a File/ Rank struct (strong typedef rather). Now all places are converted yet.
• NN index lookup for a given move is moved to neural/encoder
• Move now has en passant and castling bits, but it also simplifies movegen a bit.
• The main reason for the castling bit is to be able to convert a move to a string without knowing the position.
• Bitboard.cc had no code left, was removed.

Next planned steps in this area (before rewriting the search):

• Make Move always be "from white perspective", e.g. e7e5 by black remains encoded as e7e5.
• Combine Position with ChessBoard
• Try changing intrinsics to C++ std:: intrinsics.

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While we don't have unit tests (and we will after the refactoring!), I'm doing the manual testing now:

• Search from white/black positions (caught a bug!)
• Ponder as white/black (caught a bug!)
  Will also check the selfplay and PGN book.