`map_err` and other error-only methods

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.