Optimize and make Modint safer

[Matistjati]

The old modint was slightly footgunny because it didn't normalize in the constructor, so you could get un-normalized Mod structs if you passed numbers outside of $[0,mod)$. It's quite easy to miss this, especially if the input may contain negative numbers. To avoid performance regression from modding in the constructor, I add an "internal" constructor. The code is noticably faster for + and -. * should be about the same. The number of non-whitespace characters increases by 5.

• Another Query on Array Problem: 0.63s -> 0.45s. The function poly is nicer after the rewrite.
• Dark Alley ~0.49s -> ~0.47s. After the rewrite, we don't need to mod our answer before cout:ing

[simonlindholm]

Suggested change

	if (!e) return Mod(1);
	if (!e) return 1;

also works, right?

[Matistjati]

Yup, pushed