Implement BufMut for RefMut

[jkbpvsc]

Implementing BufMut for RefMut

[Darksonn]

I don't think people normally implement traits on RefMut. For example, the standard library forwards many traits to &mut T and Box<T>, but none of them are forwarded to RefMut. Generally, I would expect that this is because you can easily deref the RefMut<T> to a &mut T and use the impl on mutable references instead.