`map_err` and other error-only methods

[Kinrany]

Feature request: add methods like map_err that work on errors only, similar to map_ok.

To match the existing _ok methods, that would be:

• filter_map_err
• filter_err
• flatten_err
• fold_err
• map_err

Out of these five, map_err and filter_err seem the most obviously useful.