no_std support

[Benjamin Sago]

This library's singular dependency, num_traits, has recently added support for compiling without std using Cargo features. Given that this library doesn't do anything that complicated, it's in a perfect position to follow suit, and allow the computations to be performed without the parts of Rust's standard library it doesn't actually use.

I've marked this as "help wanted" because it'll be the first time I've de-stdded something.