(Mainly) Sudoku improvements

Hi y'all. I've been doing a lot of reading on sudoku in general because the mathematics behind it is fascinating. On that note, I've managed to improve the generation algorithm quite significantly, so difficulties above Easy should no longer feel so personally hateful towards you sometimes.

Alongside that, I've managed to construct a really basic hint system. It'll give you hints for extremely obvious things you've missed, and there's an option to highlight the number it thinks you should put in the cell. It's by no means perfect, so let me know if it leads you astray.