Added the bisectors method to Polyline

Generates angle bisectors while properly handling closed Polylines
Added Polyline#left_bisectors

The previous commit broke #outset because outsetting requires left bisectors instead of always-inward-pointing-bisectors (the regular kind).
Better offsetting of closed Polylines

The number of edges didn't match the number of bisector pairs when offsetting a closed Polyline because the closing edge pair wasn't being faked properly
Fixed a floating point rounding sensitivity when offsetting Polylines

Edges within an epsilon of horizontal were being detected as non-horizontal, but were rounding to horizontal in subsequent operations. Switched to using the vector with the largest y-component instead the one with a non-zero y-component.
Updated Polygon to take advantage of the floating point fix in Polyline

Now Polygon uses more of the Polyline implementation
Fixed outsetting of a concave Polygon with multiply-intersecting edges

Prohibit wrap-deleting for collinear intersecting edges
Documentation tweaks in Polyline
Added close and close! to Polygon and Polyline
Added reverse and reverse! to Polygon and Polyline