Return slices, rather than `Vec` references, in addresses

Return slices, rather than `Vec` references, in addresses

Its a bit strange to return a reference to a `Vec` in Rust, when a
slice is really intended as the way to do so. Worse, the bindings
don't know how to map a reference to a `Vec` (but do have code to
map a slice of `Clone`able objects).

Here, we move `NodeAnnouncementInfo::addresses` to return a slice,
though to do so we have to adapt the `WithoutLength` `Writeable`
impl to support slices as well.