Update: Introduce mutation tracking query helper

[smokku]

This updates #92 for latest master

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[zakarumych]

User should be able to convert Mut<'a, T> into &'a mut T, which is now impossible as there's only DerefMut which returns mutable reference with lifetime limited by Mut value itself.

[Ralith]

Off the cuff, I see no reason why there couldn't be a fn into_ref(self) -> &'a mut T.

[zakarumych]

Exactly

[Ralith]

Closing in favor of #366, which is more ergonomic and should perform better in most cases.

[smokku]

It would be prudent to close this after #366 is merged.