Fix edge cases in chess

[jathak]

The chess app that I wrote back in Fall 2014 has survived with minimal changes for 3.5 years. However, it still doesn't support the following features:

Compiled from the old interpreter (note, the repo is private to staff, so the links may not work):

• can't castle in chess
• no en passant in chess either
• chess demo doesn't detect checkmate

There's probably others too (underpromotion, for instance).

I don't really know the rules to chess that well, so I don't think I'll ever end up fixing them. This is low priority, but if anyone wants to work on it, the code is here. @sequoia-tree?