`src/listing.rs`
===

* Add `EntrySize` struct
* Impl Display for EntrySize
* Change `Option<bytesize::ByteSize>` to `EntrySize` in `Entry`
* Add Entry::Directory size calculation utilizing WalkDir
* Sort directory listings by always putting directories at the top and
  sorting files and directories based on their own semantics (ie. entry
  counts for directories and bytes for files)

`src/renderer.rs`
===

* Change `entry_raw` from using `if-let` to just operating on `entry.size`

`Cargo.lock` & `Cargo.toml`
===

* Remove `WalkDir`
* Add `fs_extra`
* Add `OnceCell`

`src/listing.rs`
===

* remove `use walkdir::WalkDir`
* add HashMap, Arc, Mutex, OnceCell, log::warn, and `FILE_SIZE_CACHE`
* Cargo fmt

`src/listing.rs`
===

* Cargo fmt
* Replace `WalkDir::new(entry.path()).into_iter().count()` with
  calculating directory size using `fs_extra::dir::get_size`
  * If `get_size` fails, fall back to `std::fs::read_dir` counting