Remove fmt::Display from Position and move display to a DiagramFormatter type that implements that trait and writes a position to a formatter.
This method does an iteration over all piece colors and shapes to find the piece on a given square.
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.
