fix: Arborist code cleanup

[wraithgar]

A bunch of tiny tweaks to Arborist, in service of eventually cleaning it
up and removing the mixin approach.

Most of the fixes are like before: removing symbols in favor of private
attributes so we can easily find where state is being shared. There is
also some shuffling of single-use methods into larger classes.

[wraithgar]

Removing or severely limiting the number of options we can pass to methods will drastically improve the code complexity here but that's a semver major.