btree: Reliance on structure layout

[tbu-]

The function collections::btree::node::Node::as_slices_internal_mut relies on structure layout of two relatively unrelated structures to be the same, which is considered a bad thing™:

#[inline]
pub fn as_slices_internal_mut<'b>(&'b mut self) -> MutNodeSlice<'b, K, V> {
    unsafe { mem::transmute(self.as_slices_internal()) }
}

[apasel422]

I'm working on this.