Ignore clippy::large_enum_variant lint on State

```
error: large size difference between variants
   --> src/lib.rs:321:1
    |
321 | / enum State {
322 | |     Idle(Option<std::fs::ReadDir>),
    | |     ------------------------------ the largest variant contains at least 624 bytes
323 | |     Busy(blocking::Task<(std::fs::ReadDir, Option<io::Result<std::fs::DirEntry>>)>),
    | |     ------------------------------------------------------------------------------- the second-largest variant contains at least 8 bytes
324 | | }
    | |_^ the entire enum is at least 624 bytes
    |
    = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#large_enum_variant
    = note: `-D clippy::large-enum-variant` implied by `-D warnings`
    = help: to override `-D warnings` add `#[allow(clippy::large_enum_variant)]`
help: consider boxing the large fields to reduce the total size of the enum
    |
322 -     Idle(Option<std::fs::ReadDir>),
322 +     Idle(Box<Option<std::fs::ReadDir>>),
    |
```

Author: taiki-e

src/lib.rs

@@ -318,6 +318,7 @@ pub struct ReadDir(State);
 /// The state of an asynchronous `ReadDir`.
 ///
 /// The `ReadDir` can be either idle or busy performing an asynchronous operation.
+#[allow(clippy::large_enum_variant)] // TODO: Windows-specific
 enum State {
     Idle(Option<std::fs::ReadDir>),
     Busy(blocking::Task<(std::fs::ReadDir, Option<io::Result<std::fs::DirEntry>>)>),