topojson.{feature,mesh} without copying points.

[mbostock]

One of the biggest performance costs in topojson.feature (and topojson.mesh) is calling array.slice on every point. There are times when this is needed, but in many cases it would be perfectly reasonable to modify the coordinates in-place when applying the transform, and to avoid the copy when creating a feature or mesh. We should design an appropriate API to enable this faster path.

Moved from topojson/topojson#215.