Victory: Refactor `VictoryPie` and `Slice` so that slices can be more granularly controlled

[boygirl]

Currently it is not possible to define radius, innerRadius or labelRadius for individual pie slices because the pathFunction that is provided to each slice already has those values pre-defined. Instead, we should provide a partially applied path function that gets calculated for each slice. We should also let radius, innerRadius and labelRadius be defined as functions of data. New radius, innerRadius props should be added to Slice, and labelRadius should be added to VictoryLabel

[boygirl]

Update:

The Slice primitive used by VictoryPie now takes radius, cornerRadius, innerRadius, padAngle, sliceStartAngle and sliceEndAngle props. Each of these props may be given as number or a function of datum and active. padAngle, radius, cornerRadius and innerRadius will be passed down from props on VictoryBar, but sliceStartAngle and sliceEndAngle must be defined directly on the Slice instance, These values should be given in degrees. To make these values easier to use as functional props, startAngle, endAngle, and padAngle (in degrees) are added to each datum passed into Slice. (If your data already has these properties they will _not_ be overridden)

Breaking Changes
The Slice primitive will still take a pathFunction prop, but this prop will no longer be provided by VictoryPie. This will not be a breaking change for most users. This will only affect users who were wrapping the Slice component and making use of the pathFunction prop provided by VictoryPie Users who were providing their own pathFunction prop to Slice should not be effected.

[boygirl]

Example:

<VictoryPie
  theme={VictoryTheme.material}
  events={[{
    target: "data",
    eventHandlers: {
      onMouseOver: () => ({
        mutation: (props) => ({
          radius: 135,
          sliceStartAngle: props.datum.startAngle + 5,
          sliceEndAngle: props.datum.endAngle - 5
        })
      }),
      onMouseOut: () => ({
        mutation: () => null
      })
    }
  }]}
/>