Take account of holdings in book key

Take account of holdings in book key

The board is numbered by rank, for comptibility with Polyglot format,
starting at a1. We then continue counting in the holdings 'files', in
up-rank direction, each holding spanning the full board height (even if
it is not fully used). Firstthe left (black) holdings, then the right,
Even in Shogi this limits the square number to < 99, which is below the
absolute limit of 128 (where we would run out of hash keys).