A handful of small changes I've made

I think the only nontrivially interesting change in here is in https://github.com/wiml/felix/commit/a9a5515444ee4d9efac659acb651602bc08ab707 , which changes some of the arithmetic used in compact linear types to be unsigned. The values are natural numbers, so an unsigned type makes more sense; but also unsigned division and modulus operations are faster than signed on the architectures that I know of.