When generating the `OutEvent` for a `NetworkBehaviour`, add the `where` clause of the
`NetworkBehaviour` `struct` to the generated `enum`.

When generating an `OutEvent` `enum` definition for a user, derive `Debug`
for that `enum`.

Why not derive `Clone`, `PartialEq` and `Eq` for the generated `enum`
definition?

While I think it is fine to require all sub-`OutEvent`s to implement
`Debug`, I don't think the same applies to traits like `Clone`. I
suggest users that need `Clone` to define their own `OutEvent`.

…eneric-connection-management