TODO (sortowane priorytem):
- [x] Iter
- [ ] IterMut
- [x] IntoIterator
- [x] ExactSizeIterator
- [x] FusedIterator
- [x] Keys
- [x] Values
- [ ] ValuesMut
- [x] Index
- [x] PartialEq
- [x] Eq
- [x] Hash
- [x] FromIterator
- [x] Extend
- [?] append (BTreeMap::append)
- [ ] postac rekurencyjna => iteracyjna
- [x] remove
- [ ] Entry