intermediate work

New EdgeCollection, a fix for EdgeView->nex/prev auto generating
nested ternary conditional, and an initial attempt to derive
"straight skeleton" offsets using a new EdgeView-derived class
EdgeViewSkel.
tuned to do completely centered thinwall path up to 2x width
instead of following part contour between 1x and 2x width
because the all-centered case avoids complexity better handled
by future dynamic flow work
intermediate work

mostly adjusting the left-right discriminating strategy to handle
cases that come up in some of the models for user thinwall issues
have medial axis fragments go deeper into non-thin region
for outer perimeter - otherwise they come up short and don't
merge with the perimeter for cases like the "rake" in #281
couple bug fixes, starting to work as expected on most issue test models

reducing code to approach that's working