`Dot` becomes `FixedDot` and `LooseDot`.
`Seg` becomes `FixedSeg`, `HalfLooseSeg`, and `FullyLooseSeg`.
`Bend` becomes `FixedBend` and `LooseBend`.
The fixed variants differ from the loose variants by being unchangeable
for the router (though probably will be pushable in some cases, but
that's for later). So typically tey're going to be the initial
conditions.
For now only the fixed variants are used even if actually loose, to
split this change into several commits.