Add named map support

[overdrivenpotato]

This should make it possible to share maps between processes. On POSIX systems, the shared memory segment is unlinked when the first map is dropped, maybe there could be a way of keeping track of how many maps are open?

[danburkert]

Sorry for the long stagnation on this, I wanted to let you know I'm still thinking about the best way to get this merged. Having shared memory support would be awesome, but my current thinking is that it may be better to have it as it's own type rather than overloading the Mmap type. I admit I don't have a lot of experience with the shared memory APIs, so this intuition may be way off. I'm plan to dig into this more and explore the possibilities.

[dtolnay]

Can this be implemented in a backward compatible way after 1.0 or do we need to be thinking about this now?

[danburkert]

I think this has bitrotted at this point, and It's never really been clear if shm fits in this crate or not. Accordingly, I'm going to close for now. We can start a new issue/PR in the future if someone wants to champion this.