Simplify a lot of the logic in DataProvider by inverting the source of
truth for Series and Collections. This change moves them from immutable
props on DataProvider to separate children components which report their
configuration (through the DataProvider's Context) into DataProvider's
state. This allows DataProvider to be responsible for watching a lot
fewer props and move the logic out to the separate components.

Additionally, this ensures that the properties for Series and Collection
are centrally-documented, due to them using TypeScript for their
implementations. This also comes with some nice syntactic sugar around
the grouping of Series into Collections (however, this is not required).

Another benefit of this cleaned system is that the system can guarantee
that Series and Collection objects which leave DataProvider are
fully-populated. This will allow us to clean up default props littered
throughout the codebase, for things like strokeWidth and color.

Finally, in order to ease this massively-breaking API change, a thin
backwards-compatibility shim has been introduced into DataProvider.
However, this is not intended to be permanent and should be removed
before the 0.5.0 stable version is released.

custom accessor support is coming

This required moving the calculated domains into the state

This leads to a crash if there aren't any series at all. Will fix later.

Things went sideways with the domains, but I think I properly fixed it
for real in the Scaler. I hope.

Because hey what's another commit on this branch, right?

Lordy.

…es-data-flow

It should be shared between Series and Collection

These are already covered by the Scatterplot stories but hey here we go

…es-data-flow

This required an update to Scaler where we need to handle the series
props changing because they come in late.