This simplifies some of the `Config` structure, in particular this
means:
Parameters which are meaningfully equivalent longer stored in
an `Option<T>`, an example of this is `channels`. If you don't
want to join any channels you simply leave it as empty instead.
In effect, `None` is behaviorally equivalent to `vec![]`.
We don't allocate when accessing certain configuration options.
For example, when accessing `channels` we used to allocate a
vector to handle the "empty case", we simply return the slice
corresponding to the list of channels instead.
We skip serializing empty or optional configuration fields.
From a deserialization perspective this is already something
that was mostly supported through use of `Option<T>` and
`#[serde(default)]`.
