Support no_std targets in Rust library

[rblaze]

Some embedded targets are no_std and lack math functions for f32. This pull request adds support for these targets by using the libm implementation of the functions.

Tested on target = "thumbv6m-none-eabi"

[sdmueller]

Hi @rblaze

Thanks for your contribution!

Just one question as I am not that well versed in the Rust ecosystem, what is the motivation to remove/ignore the Cargo.lock?

[rblaze]

This probably should be a separate pull request, let me split it away later today.

The lock file doesn't make much sense for libraries: library consumers will ignore it anyway. The only thing that will use Cargo.lock is library's own tests. Having it checked in lessens the support burden if you have autotests in CI pipeline, but pushes the "build fails with the latest dependencies" events to the consumer. Matter of choice, up to you how you want this to go.

Some details are available in https://doc.rust-lang.org/cargo/faq.html#why-have-cargolock-in-version-control and rust-lang/cargo#315

[rblaze]

Removed Cargo.lock from gitignore. The file still changed because of the new dependencies.

[sdmueller]

Thanks a lot for your contribution!