Remove fn unwrap_mut

[dhardy]

• Added a CHANGELOG.md entry

Summary

@newpavlov's suggestion: remove this fn

Motivation

UnwrapErr(rng) / UnwrapErr(&mut rng) are obvious counter-parts; rng.unwrap_err() vs (&mut rng).unwrap_err() are less obvious counter-parts.

This seems fine to me. Counter-motivation would be convenience and potentially avoiding the need to import UnwrapErr, but I don't see huge numbers of users of this anyway.

Alternative

Add fn unwrap_mut(&mut rng) -> UnwrapErr(&mut rng).

[newpavlov]

It may be worth to mention in the UnwrapErr docs that it can be used as UnwrapMut(&mut Rng) where Rng is ?Sized.

[dhardy]

@newpavlov I added doc examples