Move position.rs to the position module and create a mod.rs.
Do the same for bitboard.rs in the bitboard modules.
Export Color, Piece, Position, and Square and use crate::Thing directly instead of referring to the symbol in the nested modules.
This struct implements an Iterator that returns the positions of the 1 bits in the u64.
BitBoard takes this iterator and maps it into Squares in the pieces() method.
This is laying the groundwork for an iterator over the pieces in a Position. 👀
Add some positive tests for these shifts. Negative tests, and boundary tests are still to come.
Implement at custom fmt::Debug for BitBoard so it prints binary.
