Add unmaintained multi_mut

[pinkforest]

Fixes #1531

It has no dependants so I think unmaintained can do it ?

Ping: rust-lang/rust#39155

[Shnatsel]

Thanks for putting this together!

I'd consider "unsound" a stronger warning than "unmaintained", so I'd prefer to go with that. "Unmaintained" was originally created for a crate with no known issues but maintainers MIA, rustcrypto.

Any objections? Should we update the maintainer's guide with that?

[pinkforest]

We could have combined status for unmaintained and suspect unsound we can't get the maintainer input on ?

Otherwise it's split between two advisories or have this potential source of confusion ?

I don't mind using unsound either though.

[Shnatsel]

I think an advisory without a patched version is a strong enough signal that the crate isn't maintained well.

@tarcieri thoughts?

[tarcieri]

You can file separate advisories for both issues