Support for “System V”-style shared memory?

[astrogewgaw]

I was looking for a way to access shared memory with Rust’s memory safety guarantees built-in, and I found this crate. However, it only supports the newer POSIX-style shared memory, and not the older “System V”-style shared memory. Would adding support for it be something you might consider adding to this crate @elast0ny? I think doing so should be straightforward, but unfortunately I don’t have the necessary skills to make a PR.