markGray's edge visitor kept a stack frame on every call, though most
edges end at an object that is skipped or already gray. Those now return
without one, and the first visit and skipped cross-generation edges
tail-call out of line, as the scan visitors already do.
The walks no longer call the dense-array walker for an empty array. The
routing of nursery candidates past bypassed levels inlines its bypass
test, and the end of each pass tests its work vectors' capacity inline.
What is collected, and when, is unchanged.
Chris Tate
c1b413e5cc