Rust: fix/ignore clippy warnings
Rust: fix/ignore clippy warnings

Summary: These lint warning have been showing up for me locally when working on another project that depends on watchman.

Differential Revision: D20469325

fbshipit-source-id: 63ac4ebbd4c8d3da1a81a906eebc1b7c7db27cd5

facebook/watchman,nodakai/watchman,nodakai/watchman,nodakai/watchman,nodakai/watchman,facebook/watchman,nodakai/watchman,facebook/watchman,facebook/watchman,facebook/watchman,nodakai/watchman,facebook/watchman,nodakai/watchman,facebook/watchman,nodakai/watchman,facebook/watchman,nodakai/watchman,facebook/watchman