Fix UB and a lot of small emprovements

[WaffleLapkin]

• fix UB
• move to edition 2018
• add no_std support
• remove unsafe code
• impl Display for {ByteStr,ByteString}
• adjust docs
• clippy

[FraGag]

Thanks for the pull request!

I'm not sure about the addition of the Display impls. Per the documentation, "[...] Display is for user-facing output [...]", and escape sequences (\xNN) are not what I would consider "user-facing". That's why I omitted the Display impls in the first place. However, for strings that are "known" to consist only of printable ASCII characters (in the range \x20 to '\x7E`), then I can see that this impl would be nice to have. Do you have such a use case, or do you have another reason for adding these impls?

If we do add the Display impls, then I would appreciate it if you could add tests for them. You may just copy the debug_* tests, name them display_* and switch them to testing the Display impl.

[WaffleLapkin]

The "user" may be a user of some programming tool, so I think that it's reasonable to provide Display impl.

Do you have such a use case

I've found your crate while discussing weird output of another crate (or app? I forgot),
so while I don't have such a use case, others probably do.

I'll add tests.

[WaffleLapkin]

@FraGag would you be able to review/merge/release this? Your crate seems to be still used (around 6k monthly downloads), so it would be nice to fix UB in it.