Changes since 1.0.1:
- Added the `io_safety` feature, which implements various conversions to
  and from the `OwnedFd` and `BorrowedFd` IO safety types introduced in
  Rust 1.63. This feature is currently off by default to avoid bumping
  the MSRV too aggressively, but someday we'll bump the MSRV, these
  conversions will be implemented unconditionally, and this feature will
  become no-op.