Should there be a lineSegment primitive?

It's often awkward and confusing to use `polyline` to get a single line. Perhaps there should exist primitives `lineSegment` and `thickLineSegment` that take exactly two coordinate pairs as arguments (or just four numbers?), and produce a line segment.

Note that the other polygonal-style shapes: `polygon`, `curve`, and `closedCurve` and their thick and solid variants, depend on having at least three points, so there would be no need to do the same there.

まず、Haskellコードベースにある既存のpolyline、polygon、curve、closedCurveプリミティブを読みます。2つの座標ペアまたは4つの数値が既存のAPIに適合するかを判断し、対応するlineSegmentとthickLineSegmentの動作を定義して、単一のセグメントでpolylineが不要になることを確認します。

索引モデルが issue の本文から書いたものです。