Helper functions added
updateExtremity fixed
`go` replaced by `walkLeaf`' in lookupGT
Recursion in `size` replaced by `foldl`
`deleteExtremity` introduced
Benchmark for `size` added
Some `case`s replaced by `maybe`
`filterWithKey`, `mapMaybeWithKey` and `mapWithKey` unified
Explicit children setters replaced by setLeft/setRight/link
`second` replaced by <$>
`direction` replaced by `choose`