Implement thiserror::Error for a bunch of error types, and remove string errors
from the implementation of the command handler in explorer.
Clean up parsing of basic types all over the place.
Update Cargo files to include thiserror and anyhow.
Well… all the tests except the Peter Ellis Jones tests.
Pass around whole EnPassant types instead of pulling out just the e.p. square.
Make sure that Castling moves have their target and origin squares populated.
Add a color field to the Castle move style to make this possible.
