Added a 'points' attribute to Polyline

Everything else has one
Added support for slicing Point and Size
Override Polyline#closed? in RegularPolygon

A regular polygon is always closed
Added a points attribute to RegularPolygon

It already had #vertices, so only an alias and a test were needed.
Added a ‘first’ method to the one and zero placeholders

This works around the need to support a map method that works for an infinite sequence
Fixed rotation of PointZero
Added min, max, and minmax to Triangle

Stolen from Polyline
Added closed? to everything that’s always closed