Implement mut_chunks() method for MutableVector trait.

mut_chunks() returns an iterator that produces mutable slices. This is the mutable version of the existing chunks() method on the ImmutableVector trait.

EDIT: This uses only safe code now.

PREVIOUSLY:
I tried to get this working with safe code only, but I couldn't figure out how to make that work. Before #8624, the exact same code worked without the need for the transmute() call. With that fix and without the transmute() call, the compiler complains about the call to mut_slice(). I think the issue is that the mutable slice that is returned will live longer than the self parameter since the self parameter doesn't have an explicit lifetime. However, that is the way that the Iterator trait defines the next() method. I'm sure there is a good reason for that, although I don't quite understand why. Anyway, I think the interface is safe, since the MutChunkIter will only hand out non-overlapping pointers and there is no way to get it to hand out the same pointer twice.