fd9ca0c25e
Implement `core::slice::IterMut::as_mut_slice` and `impl<T> AsMut<[T]> for IterMut<'_, T>` As per [the zulip discussion](https://rust-lang.zulipchat.com/#narrow/stream/219381-t-libs/topic/.60std.3A.3Aslice.3A.3AIterMut.3A.3Aas_mut_slice.60), the `AsMut` impl has been commented out, with a comment near the `#[unstable(...)]` to uncomment it when `as_mut_slice` gets stabilized.