Add map and try_map methods to BoundedBTreeMap (#11869)

* Add map and try_map methods to BoundedBTreeMap

* Undo changes to basic_authorship.rs

* Remove unwrap and use unchecked_from instead

* Add iter_mut() method

* Remove map functions and add docs to iter_mut

* fmt

Co-authored-by: Shawn Tabrizi <shawntabrizi@gmail.com>