Tweaking concat! macro to accept bytes string

[KokaKiwi]

Hi, I'm working on tweaking a bit the concat! macro in order to make it accepting bytes string as well (and making the output dependent of the first argument's type), but I don't know if it needs a RFC or not, so I ask it here (after some guy's advices on IRC).

Also, if you have some ideas about better ways to accept bytes string in concat! macro (I was thinking about making a concat_bytes! macro instead for example), feel free to tell me. :)

[nodakai]

Interesting idea. Any updates?

[golddranks]

I just happened to need this today!

[asayers]

Looks like @KokaKiwi put it in a crate in the end.

[Centril]

Closing in favor of #2509.