Unite as_ref and as_mut features

[MegaBluejay]

First suggested here

Synopsis

AsRef and AsMut derives are effectively the same regarding implementation, so using separate features for them doesn't make sense.

Solution

Use the as_ref feature for both derives

Checklist

• Documentation is updated (if required)
• Tests are added/updated (if required)
• CHANGELOG entry is added (if required)

[MegaBluejay]

We could keep the as_mut feature around as a synonym, making this a non-breaking change.
As far as I understand removing it from the public interface is part of the goal here, so I didn't.

[tyranron]

@MegaBluejay we're going to 1.0.0, so breaking changes are okay now, not later. I think that keeping as_mut as synonym will make even more confusion to library users. It's okay to have just as_ref, just deref, and so on...