Tweaking concat! macro to accept bytes string #1187

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. :)