Add correct vs current value tracking in cells; simplify value entry logic

added crude MVP for entering digits into cells

updated crossterm version

upgrade tui to 0.17 (as well as rust)

removed some unneeded pubs

moved formatted sudoku grid into StatefulGrid

tweaked colors, added style for selected clues

added cell styling, including distinct color for clues

added getter for element clue status

no need to totally reconstitute the cell when styling

made debugging output a little more elegant

temp removed col functions since they're broken and not necessary yet

fixed sudoku board not displaying correctly; elements now track if they're a clue

move cursor out of grid when quitting so board can be viewed

removed superfluous spacing