Leave setting values as `Undefined` rather than coercing to `nil` for un-set settings

[timriley]

This is something @solnic mentioned a while ago as a change he'd like to see for 1.0.0. Opening this so we can have the discussion as whether this is something we still want.

[timriley]

Related to this would be changing the expected signature for setting value processor blocks: they must be prepared for their value argument not to be provided for undefined values, so a processor block like this:

-> v {
  # some logic
}

Must become:

-> v = "default" {
  # some logic
}

When a setting is defined with a default, however, this can just be kept as -> v { }, since the default value will always be passed.

This arrangement would encourage the authors of the setting definitions to make sure they're properly handling the case of a non-provided value in their processor blocks if no default is otherwise defined.

[flash-gordon]

I'm in favor. This is definitely the place that can benefit from Undefined. I wish we didn't have to break things but this is for the greater good :) Mb we could have prev behavior ... configurable (but not default).