Explicit set of left/right replaced by setLeft/Right
Explicit set of both children replaced by `link`
`case` replaced by `maybe`
`split` simplified
Explicit selection before/after replaced by `select`
`findLeaf` introduced
`minKey` removed from submapTypeBy
`compareSplits` introduced
`internal` function introduced'
`calcDirection` transformed to `chooseByDiff`
Redundant constructor arguments removed
`ifMaybe` introduced