restore no_std after diman_lib was split out

[jedbrown]

When we added no_std (https://github.com/Tehforsch/diman/pull/60), crates/diman_lib did not yet exist. It's now missing the no_std annotation, which has allowed tests to pass (since the tests are all running for architectures that support std). Rust does not prevent no_std crates from calling crates that use std (this is confusing).

Adding no_std to diman_lib moves the dependency on num-traits into diman_lib and we drop the ratio module when we don't have either std or num-traits-libm (since we need powf for it to work).

I've tested this with --target=nvptx64-nvidia-cuda.