Some `Debug` and `Display` impls are hidden behind the `std` feature

[aleasims]

I found out that a lot of Debug and Display impls are gated by std feature (e.g. here, here, and in many other different places). I'm wondering was it done on purpose or just by a mistake?

Those traits are useful in no_std crates. For example, I'm using your library in no_std environment and I'm not able to do things like:

fn error_to_string<T, E>(error: TrieError<T, E>) -> String {
    error.to_string()
}

I'd appreciate any comment on that.

Also I'm ready to help cleaning up the no_std support in all the crates to get rid of "hacky" solutions with rstd and to make it look more "sound". Does it sound reasonable to you?

[aleasims]

Here is an example PR of what could be done to improve no_std compatibility:

• Make hash-db always a no_std library #229