Fillet (round corners) operation

Graphite needs a Fillet node, and it turns out this is somewhat nontrivial and would probably be better handled by Kurbo, since this seems like a commonly useful operation. So I thought I would request it here, since we've had trouble even finding any relevant literature about it that we could use as reference for our own implementation. To those unfamiliar (it's most common in CAD), this entails inscribing a circle at sharp corners.