The split face code relied a lot of the edges added to the weld context.
But with 113004687d, some edges are removed from context. And even more
edges can be removed (reducing loops and arrays).
Removing the dependence on edges decreases the array and time spent in
loops.
Furthermore, the corner edge array should not be needed for the
merge by distance operation.
So this commit redoes the face/loops iterator which previously
needed the `loop_next` member to skip loops depending on the previous
one (possibly linked to the edge whose vertex will be merged) and now
uses the `switch_to` member to switch loops instead of skipping and
depend on the former.
6ec842c43c