This is tag v0.3.9 with cherry picks
  - 47dc5fe Cast function item to function pointer in order to appease compiler.
  - f2a6b6d Only deny warnings in test

and changes to Cargo.toml necessary to publish the crate.

This is needed to fix builds of dependent crates for nightly Rust. For
more info, see
  https://github.com/rust-lang/rust/pull/34198#issuecomment-225283945