refactor board, cleanup

[clemenscodes]

• refactor(board): generic encode trait implementation
• refactor(board): automatic trait implementations for moves
• refactor(board): piece symbols
• refactor(board): fen constants associated with impl
• refactor(board): pieces macro, remove is_illegal_move trait method

[codecov-commenter]

Codecov Report

Merging #17 (d9f7b0d) into main (6bf7279) will increase coverage by 0.96%.
The diff coverage is 57.65%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #17      +/-   ##
==========================================
+ Coverage   64.03%   64.99%   +0.96%     
==========================================
  Files          62       61       -1     
  Lines        4154     3931     -223     
==========================================
- Hits         2660     2555     -105     
+ Misses       1494     1376     -118     

Components	Coverage Δ
gui	∅ <ø> (∅)
api	54.43% <0.00%> (-2.15%)	⬇️
bitboard	95.65% <ø> (ø)
board	65.33% <60.95%> (+1.47%)	⬆️
controller	0.00% <ø> (ø)
draken	∅ <ø> (∅)
engine	∅ <ø> (∅)
game	47.40% <ø> (ø)
model	0.00% <ø> (ø)
uci	0.00% <ø> (ø)
view	0.00% <ø> (ø)