Changes:
````
new_ret_no_self: add sample from #3313 to Known Problems section.
Support multiline comments and hopefully fix panic
Check for comments in collapsible ifs
Resolve ICE in needless range loop lint
RIIR update_lints: Update changelog links
Rename if_let_redundant_pattern_matching to redundant_pattern_matching
Add lint for redundant pattern matching for explicit return boolean
Fix issue #3322: reword help message for len_zero
Simplify manual_memcpy suggestion in some cases
Fix dogfood
Update known problems for unnecessary_fold
RIIR update_lints: Replace lint count in README.md
Rename `active_lints` to `usable_lints`
Add comment on WalkDir vs. fs::read_dir
sort_by -> sort_by_key
Some more documentation for clippy_dev
Use `WalkDir` to also gather from subdirectories
Avoid linting `boxed_local` on trait implementations.
Website: Make lint categories linkable
Restore clippy_dummy's placeholder name
Swap order of methods in `needless_range_loop` suggestion in some cases
Revert "Exclude pattern guards from unnecessary_fold lint"
Exclude pattern guards from unnecessary_fold lint
````