Add zero-copy make_mut

[Sytten]

Supersede #687
Closes #611

This is my first time contributing to bytes and this is quite a change, do let me know if you want more tests.

[mattfbacon]

Why is x.make_mut() not the same as x.try_into_mut().unwrap_or_else(|| BytesMut::from(&*x))?

[Sytten]

I mean I guess we could but we already have to handle the copy case in each to_mut vtable function so I dont really see the point in not using that. Plus it there is a small chance that a concurrent thread released the shared since we use acquire vs relaxed ordering.

[Sytten]

Sorry I didnt have to work on it more, I will try this week. @braddunbar if you have for a review I would appreciate 🙏

[braddunbar]

Oh, I missed the notification. Thanks for the ping! I'll take a look.

[Sytten]

@braddunbar @Darksonn Gentle ping :)
I added the comment and an odd test. Let me know if we want to add more tests.

[Sytten]

@Darksonn Done the improvements

[Sytten]

@braddunbar @Darksonn Ready for another round :)

[Darksonn]

Mostly LGTM.

[braddunbar]

I spent some time combing over this today and I can't find anything to complain about. Thank you @Sytten!