We were using exact floating point comparison to map out the faces
and vertices of HEALPix pixels. When a vertex that is shared
between two faces had slightly different floating-point coordinate
representations in different faces, contours were lost.

In the new approach, we identify vertices by the set of faces that
share them.

Also, the `ligo-skymap-contour` tool got 10x faster thanks to an
optimization that allows us to skip most pixels that are on the
interior or exterior of the contour.