usize vector types

[bitshifter]

There have been a few requests for USizeVecN.

I'm not sure if this needs the full unsigned int vector interface or not although it's probably easier to make it the same as the other unsigned integer vector types. So far isize vectors have not been requested.

Discussed in #261

^{Originally posted by sarkahn September 3, 2021}
I was wondering if there's any chance we could get a "SizeVec" or "USizeVec" (Vec whose elements are usize). I use (usize,usize) a lot to represent 2D size, I find myself wanting to replace them with UVec2 to make the math easier to read, but then I have to convert to usize to interact with array indices and such.

Naming wise maybe USVecN ISVecN would be more glam like but they also seem not super clear, the alternative being USizeVecN and ISizeVecN.