Make apply_compactify with abs_tol > 0 leave breadcrumbs along a trail of tiny revisions

[brookslogan]

Currently, if we have a long sequence of tiny (< abs_tol) revisions, all of them will be compactified away. It'd be nicer if we recorded revisions once they stray more than abs_tol away from the previous kept revision. This should be easy to do if we have iterators refactor in place #173 (comment).