Provides mappable Rc and Arc types.

```rust use mappable_rc::Mrc;

let m: Mrc<[u32]> = vec![1, 2, 3, 4].into(); asserteq!(m.asref(), &[1, 2, 3, 4]);

let m: Mrc<[u32]> = Mrc::map(m, |slice| &slice[1..=2]); asserteq!(m.asref(), &[2, 3]); ```