Any chance for make_mut()?

[cehteh]

From the Docs:

• ArcStr is totally immutable. No need to lose sleep because you’re afraid of code which thinks it has a right to mutate your Arcs just because it holds the only reference…

This makes me wonder if you strongly oppose such a feature or are open to a PR which adds this (possibly feature gated).

For the time being I made my own 'cowstr' crate, but on a closer inspection it should be possible to add mutability to 'ArcStr' in a sound way.