Implement arbitrary for CString

[ThomasdenH]

Closes #165. This implementation builds a CString from either a String or a Vec<u8>. It shrinks using the VecShrinker, everywhere making sure no null characters end up in the CString.

To prevent using as many allocations, it is built using an iterator, which is then filtered directly. I can change that if it's undesirable.

[ThomasdenH]

Is this PR acceptable? Should I change anything?

[maxbla]

@BurntSushi seems to keep pretty busy. He maintains quite a few libraries (just look at his github!). He takes a while to get around to reviewing PRs, but he often does eventually. Mentioning him with @ (as I already did) sometimes helps.

I can give you a comment in the meantime though.

I'm not convinced of the need for your ArbitraryIterator. You say it avoids excess allocations, but I don't understand how. You could do (0..).map(|_| Arbitrary::arbitrary(g)).filter(|&c| c != '\0').take(size).collect(), which I believe is equivalent to your CString code, both in final result and in number of allocations.