A generalization of d3.geoCircle with a parametric curve generator:
d3.geoCircle() == d3.geoShape().generator(t => [Math.cos(t), Math.sin(t)])
TODO: add a .rotate() method, based on:
// rotation
var r = angle * Math.PI/180,
c = Math.cos(r),
s = Math.sin(r);
return [x * c - y * s, y * c + x * s];
```
See the code at [github:Fil/d3-geo/geoShape](https://github.com/Fil/d3-geo/commit/cd6ad7a916ed99d5f74bd6004cd9ad66a67a6266)