Make libm optional in a std environment

[bantonsson]

When adding proptest as a dev-dependency it will add libm transitively no matter what, and this has noticeable effect on our criterion benchmarks.

This happens even if you add proptest like this:

proptest = {version = "1.5", features = ["std"], default-features = false}

[matthew-russo]

This got closed when i switched branch from master to main. I'll re-open against the main branch.