Voronoi bracelet

I code a `voronoi` and `bend_extrude` for [my library](https://github.com/JustinSDK/dotSCAD). Here is a demonstration about how to put them together. They will be released at the next version of dotSCAD.

`voronoi` produces a 2D [Voronoi Diagram](https://en.wikipedia.org/wiki/Voronoi_diagram). The purpose of `bend_extrude` is to replace [`bend`](https://openhome.cc/eGossip/OpenSCAD/lib-bend.html) when you have a 2D shape. `bend_extrude` is faster and doesn't produce jagged edges.