Holy heck I went on a *journey* here. Ultimately, I needed to implement my own
index-based iterator instead of using the Vec's Iterator.
This type establishes some patterns I want to carry forward to other move
generators.
1. The use of a Parameters struct to fully parameterize the move generation
per-color. That lets these types only need a single color-based branch
2. A list of move lists, one list for each of captures, promotions, and quiet
moves.
3. An index-based move iterator.
4. Separate impl for generating bitboard representations of these moves
Additional changes:
- Implement BitBoard::from_square()
- Implement a Square::e5() for tests
This class doesn't implement en passant yet. It also doesn't yet have tests for
the bitboard stuff.
BitBoard::file returns a BitBoard representing the 0-indexed file.
fmt::Display prints a grid of bits in the standard orientation (white on bottom, left to right)
Add asserts to the rank and file constructors to catch out of bounds arguments.
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.
