`DerefMut` should be `for Sized?`

[pythonesque]

This currently is not the case, so you can't impl DerefMut<T> for Foo where Foo is unsized. Note that this is distinct from whether T is Sized? (it is).

[csouth3]

Hmm, yeah I guess at first glance it seems strange that Deref is for Sized? but DerefMut is not...and this at least looks like an easy fix to me.

[csouth3]

I'm not really sure who to ping on this, maybe I should just make up a pull request and see what happens?

[pythonesque]

Yeah, probably :)

[sfackler]

Yeah, just adding for Sized? seems fine.

[csouth3]

Awesome. I'll put together a pull request now then.