Simplify Config structure

[udoprog]

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)].

[aatxe]

Thanks, this is a great change. The current setup is a hold over either from before serde had all these nice features or possibly just from before I knew any of them existed. The Options were definitely there to deal with skipping over those fields.

[ParadoxSpiral]

This reminds me of a question I had: why is the server field optional?

[udoprog]

@ParadoxSpiral it would seem to be to support testing without specifying it. Trying to use a non-mock connection without it will cause it to error. It could be done cleaner.