Make U*CString growable?

[DBLouis]

Would it be a possible to make U*Cstring growable? Adding at least a push method. It would require changing the buffer to Vec<u*> instead of Box<[u*]> and moving to NUL terminator to the end when appending. I can work on a PR.

[VoidStarKat]

It was originally based off of how std CString was implemented, but no reason it can't be changed so feel free to submit a PR