DonutChart → Donut, DonutSlice → Donut.Slice

[shawnbot]

This is a proposal to help us decide on #166. It renames the DonutChart component to Donut, moves the DonutSlice component into src/Donut.js, and makes it available as Donut.Slice. Before:

import {DonutChart, DonutSlice} from '@primer/components'
export default () => (
  <DonutChart>
    <DonutSlice state="pending" value="3" />
    <DonutSlice state="success" value="2" />
  </DonutChart>
)

After:

import {Donut} from '@primer/components'
export default () => (
  <Donut>
    <Donut.Slice state="pending" value="3" />
    <Donut.Slice state="success" value="2" />
  </Donut>
)

And if this still seems too verbose, we could ditch the Slice component altogether and allow only the data prop.

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[emplums]

Just left a few questions! I'm cool with this method but would like to see what the others think. If everyone is on board let's do a major release next week with this & the Link work in it.

[emplums]

@broccolini @jonrohan do either of you have any thoughts on this? If everyone feels good about this PR we'll use it to model other parent/child components

[emplums]

@shawnbot I feel good about merging this for now so we can have it in this week's release - was hoping we could discuss this model for parent/child elements as a team so we're all on the same page, maybe for next week's review?