Regularize Chu-Shogi piece assignment

Regularize Chu-Shogi piece assignment

The pieces in Chu Shogi have been reassigned to other internal codes,
so that juggling of images in variant cu is no longer needed. This
became possible by the flexible assignment of promotion partners.
It makes it easier to use Chu Shogi as parent variant without having
to worry about a non-stadard meaning of the -pieceToCharTable.
The built-in move generator had to be changed to work for the new
assignment.