Add safe translation for open/read/write/close

[lucic71]

This PR adds the building blocks of safe file descriptor IO translation. In Rust, C file descriptors (int), are usually translated as OwnedFd/BorroweFd. nix also uses OwnedFd/BorrowedFd inside the IO APIs, for example open is defined as: pub fn open<P: ?Sized + NixPath>(path: &P, oflag: OFlag, mode: Mode, ) -> Result<OwnedFd>

However, we cannot directly translate integers to OwnedFd/BorrowedFd. An integer can be used as both integer and OwnedFd at the same time, which Rust does not allow. For example:

int fd = -1; // plain integer
fd = open(...) // file descriptor semantics
if (fd > 0) // plain integer
  write(fd, ...) // file descriptor semantics

To overcome this limitation, allow integers to be translated as i32, and keep a global FdRegistry which contains mappings of type i32 -> OwnedFd:

pub struct FdRegistry {
    fds: Vec<Option<OwnedFd>>,
}

impl FdRegistry {
  // Creates a new i32 -> OwnedFd entry
  pub fn register(fd: OwnedFd) -> i32

  // Applies function f over the BorrowedFd: f(FdRegistry[fd])
  pub fn with_fd<R>(fd: i32, f: impl FnOnce(BorrowedFd<'_>) -> R) -> R

  // Removes the i32 -> OwnedFd entry
  pub fn close(fd: i32) -> i32
}

This way, integer operations on file descriptors continue to work, for example int fd = -1 or if (fd > 0) and functions that use file descriptor access the OwnedFd/BorrowedFd through the FdRegistry, for example:

let fd: i32 = -1;
fd = FdRegistry::register(nix::fcntl::open(...));
if (fd > 0)
  FdRegistry::with_fd(fd, |bfd: BorrowedFd| nix::unistd::write(bfd, ...))

In the following PRs I will add the remaining safe IO rules.