Composable controls

[Ralith]

If a source is wrapped in multiple filters with dynamic parameters (e.g. for #9), it can be difficult to access more than the outermost. We should provide some sort of helper to expose them equally.

[Ralith]

One viable approach for this is to crib from frunk::HCons::pluck. That will require a distinct Filter trait whose values are chained in a hlist-like construction, rather than simply wrapping sources.