Move rand to [dev-dependencies]

[cuviper]

It's only used in a few tests, not in the crate proper.

[codecov]

Codecov Report

Merging #27 into master will decrease coverage by 0.45%.
The diff coverage is n/a.

Impacted Files	Coverage Δ
src/map.rs	83.95% <0%> (-0.7%)	⬇️

[jonhoo]

Good catch! I think there's an argument that num_cpus should also be moved behind a feature flag. We'd just have to figure out a sensible value to set stride to. That would also help with #26 where num_cpus isn't available.