Feature: add `Cow` type

It might be nice to have a `Cow` type in heapless.

```rs
pub enum Cow<'a, B, const N>
where
    B: ToOwned,
{
    Borrowed(&'a B),
    Owned(<B as ToOwned>::Owned<N>),
}
```

Example
```rs
let string = heapless::Cow<'static, str, 8>;
let vec = heapless::Cow<'a, [u8], 16>;
```

But I'm not sure if this is actually a good idea because you have to allocate the entire `Owned` object anyway.