Checkpoint some broken stuff.
More brokenness.
Trivial topology generation (arcify).

Arcify is a “first-pass” topology construction that does not detect coincident
points and instead treats each input line as a discrete arc. This is a first
step towards constructing the topology. Each arc is represented as a continguous
slice of a shared point buffer; this will allow easy cutting of arcs later when
coincident points are detected.
Record every arc occurrence by point.
Slightly stronger test.
Use d3.geo.stream.
Fix for implicit closing point in polygons.
Don’t compute occurrences during arcify.

Also, don’t include point geometries in the coordinate buffer, since these are
not converted into arcs.
Progress on unifying arcs.
Delete unused file.
Delete comments.
Detect partial matches.
Use inclusive end index for symmetry.
More unification.
Another test.
Commenting.
More tests.
More tests.
More tests.
New arc extends old arc.