separate stack modification in Path into push/pop/currentNode/currentIndex/setIndex
slight tidy
refactor + comment succ/pred
comment
merge
comment
simplify away cpfPos
make copyFromKeyEnd into a local
comment
simplify
rename to Builder and NodeBuilder
get btree unit tests running in under 10m
simplify ML.toNode() and ML.ascendToParent()
fix ascend change for a post-buildFromRange world
comment/cleanup
extract MAX_KEYS
replace << >> with * / 2
cleanup