num-traits uses `std` in default enabled features

https://docs.rs/crate/num-traits/latest/features

This creates an issue since I'm using f32 and compilation fails because of missing `std`:

```
error[E0463]: can't find crate for `std`
--> /home/elpiel/.cargo/registry/src/index.crates.io-6f17d22bba15001f/num-traits-0.2.19/src/lib.rs:23:1
|
23 | extern crate std;
| ^^^^^^^^^^^^^^^^^ can't find crate
|
= note: the `thumbv8m.main-none-eabihf` target may not support the standard library
```