Support wall squares (#565)

Support wall squares (#565)

Add support for squares that are neither accessible
nor traversable, using the standard `*` notation.

* Add support for duck chess, closing #528.
* Add support for wall squares, e.g., Omicron and Gustav III chess, closing #53.
* Support petrification on capture.
* Refactor game of the amazons to use wall squares.
  * This changes both the FEN and move representation.
* Enable -DALLVARS for both pyffish and ffishjs.

Note: With this commit `pieces() == pieces(WHITE) | pieces(BLACK)`
can no longer be assumed due to the possibility of wall squares.