Micro-optimize vec.with_c_str for short vectors

One of the downsides with `c_str` is that it always forces an allocation, and so this could add unnecessary overhead to various calls. This PR implements one of the suggestions @graydon had for `c_str.with_c_str` in that for a short string we can use a small stack array instead of a malloced array for our temporary c string. This ends up being twice as fast for small strings.

There are two things to consider before landing this commit though. First off, I arbitrarily picked the stack array as 32 bytes, and I'm not sure if this a reasonable amount or not. Second, there is a risk that someone can keep a reference to the interior stack pointer, which could cause mayhem if someone were to dereference the pointer. Since we also can easily grab and return interior pointers to vecs though, I don't think this is that much of an issue.