Use `AsRef` to accept both `Bdd` and `&Bdd` in core methods

[daemontus]

In many methods for Bdd manipulation, we often expect a reference to another Bdd (like the and function). This makes the API slightly weird to use because we have to put & everywhere. However, there is a useful built in trait AsRef that we can implement in Bdd that can be used for this easily.

[daemontus]

This will not work :/