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.
Replace Position's en_passant_target_square() and en_passant_capture_square()
with a single en_passant() method that returns a new EnPassant struct that has
the target and capture squares for the en passant move.
