Lint idea: `file_name` obtained from a call to `path` obtained from a `DirEntry`

[smoelius]

Should use DirEntry::file_name instead.

DirEntry::file_name returns an OsString.

DirEntry::path returns a PathBuf (a larger allocation) and Path::file_name returns an Option<&OsStr> (a more complicated type).

As mentioned in #920, clippy_utils has expr_or_init, which can help with lightweight dataflow problems like this.

[smoelius]

The lint should apply to walkdir::DirEntrys as well, though its types are slightly different.

walkdir::DirEntry::path returns a &Path.

walkdir::DirEntry::file_name returns an &OsStr.

Thus, the "larger allocation" comment does not apply, but the "more complicated type" comment does.