Add (nightly) no_std support

[hobofan]

Fixes #10.

• Inlined the ref_eq dependency, since that crate did not have no_std support and was only a single function
• I had to add a dependency on libm, since core is currently lacking maths capabilites. See this RFC

[emilsjolander]

@hobofan Actually should we also add a test to circle ci which ensures it compiles with no_std to ensure we don't regress this later?

[hobofan]

Since it depends on nightly versions of Rust, this would make at least that portion of the tests really flakey, which is usually the reason why nightly builds are allowed to fail on Travis.

Last time I checked, support for a build matrix and allowed failures wasn't that great in CircleCI, but that might be outdated (either way I don't know enough about Circle to set it up properly).

On a related note: I'm working on cargo-nono. It's currently not very comprehensive in what it actually checks for in the the code, but I hope to soon add support for really obvious violations (e.g. use std::), and then it should also make sense to include it into CI builds to avoid regressions.

[emilsjolander]

Awesome. That makes sense. Yeah let's leave it as is for now.