Serialize to LSB bytes

Since [`RoaringBitmap::from_lsb0_bytes`](https://docs.rs/roaring/latest/roaring/bitmap/struct.RoaringBitmap.html#method.from_lsb0_bytes) lets you convert lsb bytes into a `RoaringBitmap`, it would be helpful to have an API to convert a `RoaringBitmap` back to lsb bytes. My app uses lsb bytes to communicate with other components, but works with `RoaringBitmap` internally, so I need to go both ways.