multi_mut is Unmaintained

Last release was about 6 years ago.

There is an outstanding [soundness issue](https://github.com/golddranks/multi_mut/issues/1).

The maintainer has not responded for two years to the existing soundness issue.

Rust compiler has enabled errors relating to LLVM noalias rules and may not
compile anymore where as the old compiler versions had turned these off.

The maintainer has stated:

> I will take no responsibility of undefined behaviour possibly caused by this crate.

This crate may or may not be suitable for use anymore given the outstanding issues.