channel aliases; mark.with

[mbostock]

Supersedes #719; related #708 (comment). For example (note that the data is randomly generated, but the line and dots are consistent, hence demonstrating that the aliased values are not recomputed in the derived mark):

Plot.lineY({length: 500}, Plot.mapY("cumsum", {y: randomNormal, stroke: "blue"})).with(Plot.dot)

There are still some things to figure out here, though, like what does this mean for transforms? In this implementation, the transform is re-applied in the derived mark, but I don’t think we want that; I think instead the derived mark should somehow inherit the derived data and index from the source mark without recomputing the transform. Then any new channels on the derived mark would presumably be computed on the transformed data, similar to layouts?