RadialBar: Utilize defs/fill, modeled after Bar implementation

[bdefore]

I wanted to use the patterned fill seen in https://nivo.rocks/marimekko/ for my use of ResponsiveRadialBar but noticed that defs and fill props were not expected in the RadialBarSvgProps type nor the implementation to pass them along to SvgWrapper in RadialBar the way that Bar does. This PR adds that implementation in a similar way.

Intermediate commits show I was initially pursuing this as Bar does by having bindKeys do a targetKey of data.fill (https://github.com/plouc/nivo/blob/master/packages/bar/src/Bar.tsx#L268) but it felt more appropriate to instead avoid polluting incoming data and instead target fill at the root ComputedBar. I'd be fine with either approach if one is preferred.

Note that the test suite fails on packages/scatterplot/tests/ScatterPlot.test.tsx but I believe these failures predate this proposal.

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[plouc]

@bdefore, thank you for this addition, it makes sense to do it the way you did, and it works with the arcs package!