Allow library to be use in embedded projects.

This can be tested with:
```
cargo build --target thumbv6m-none-eabi
```

Things to fix:
[ ] FFT crate doesn't support no_std. Maybe use https://gitlab.com/teskje/microfft-rs ?
[ ] Add `libm` for floats

Start by running `cargo build --target thumbv6m-none-eabi` to identify the embedded build failures. Review the FFT crate's no_std support and the float usage mentioned in the issue, then evaluate the suggested microfft-rs and libm options. Done means the target build succeeds without losing the library's DSP functionality.

Written by the indexing model from the issue text.