exact/perturb: Slight simplification
config.h: In debug mode, check if we reach OTHER_UNREACHABLE()
exact/circle_csg: Halfway towards lower degree predicates

Ray casts for depth determination now use rays at quantized y-values.
As a consequence, all circle_intersections_upwards calls share an arc
between the two compared vertices, which will allow us to reduce the
predicate degree down to six.
Merge branch 'exact' of github.com:otherlab/core into exact
exact/circle_csg: Fix indices produced by prune_small_contours

Also expose preprune_small_circle_arcs for use in the paper benchmarks.