Add const to some functions.

[poweron0102]

It just make some ::new() function to be const.

allowing it to be call on const definitions like this:

// Old
const SIZE: TilemapSize = TilemapSize{ x: 256, y: 256 };

// New
const SIZE: TilemapSize = TilemapSize::new(256, 256);

And some compile time optimizations.

[StarArawn]

I fixed the clippy issue elsewhere. I'll merge this. 👍