Allows us to write `Diagram B` instead of `Diagram B V2 Double/Float/Whatever` in diagrams programs

Currently the linear changes has made the `B` token less useful since the type `Diagram B V2 n` the n depends on `B` the backend.
This provides the ability to switch backends simply by changing the import by using `Diagram B` instead of `Diagram B V2 Double` e.g.

 * Companion branch in core