Introduce max_by/min_by on iterators
See https://github.com/rust-lang/rfcs/issues/1722 for reference.
It seems that there is `min`, `max` (simple computation of min/max), `min_by_key`, `max_by_key` (min/max by comparing mapped values) but no `min_by` and `max_by` (min/max according to comparison function). However, e.g. on vectors or slices there is `sort`, `sort_by_key` and `sort_by`.