1-d KDE transform?

(As mentioned in a few places: #948, #943, #791.)

We can look at https://github.com/uwdata/fast-kde/ for speed, and https://observablehq.com/@d3/kernel-density-estimation for a more straightforward implementation. Could be used to create violin plots, etc. Building on our 2-d implementation, the efficient approach might be to just bin (with linear binning for when a point doesn't fall exactly on a point) then blur.

An (old) experimental notebook: https://observablehq.com/@observablehq/fast-kde-and-plot/2

(2-d KDE is addressed by the density mark.)

Examples would include violin plots

(Need to add some padding.)