Use associated types in Doublets trait

Nobody has claimed this yet.

### Use `Doublets` instead of `Doublets`
This is idiomatically more correct. For example – `AsRef` and `Deref`:
```rust
let string = "hello".to_string();
// we want `AsRef<[u8]>`
let bytes: &[u8] = string.as_ref();
// we want `AsRef`
let str: &str = string.as_ref();
```
```rust
let string = "hello".to_string();
let str = &*string; // we are given `str`
```

This will also make generic storing doublets than easier.
```rust
struct Wrapper> {
doublets: D,
_marker: PhantomData,
}
```
```rust
struct Wrapper {
doublets: D,
}
// `T` is `D::Item`
```

Locate the Doublets trait and every Doublets implementation or use. Read the trait bounds and existing tests first, then update the API to use the associated Item type and verify that the project compiles and its tests pass.

Written by the indexing model from the issue text.