Refactor box drawing into separate function.
Type-correctness in read_puzzle.
Allow space (' ') to be a blank when reading files.
Use dot ('.') for blank rather than space.
Do not bold row/col headers.
Add new bit for user-supplied value.
Rename WIN to WIN_VALUE.
Add helper methods for bitfields.
Add a color for user-supplied values.
Add an explicit color for immutable values.
Use a 'visual cue' (in this case > <) instead of blinking cursor.
Bring in latest bitfields/macros.
New coloring priority: 1) highlight twins 2) color immutables 3) color user-supplied.
Rename 'val_under_cursor' to 'val_at_iterator'.