Add as_mut_inner method

[wojciech-graj]

No description provided.

[Lokathor]

This CI fail is an msrv problem. I think our msrv policy is "Stable-20", so that's 1.72 currently. I forget when mutable refs in const fns was allowed, but we could make the fn not be const if necessary.

[Lokathor]

Or we could make it feature gated, that works too!

[wojciech-graj]

Or we could make it feature gated, that works too!

Sorry, just saw this after already pushing the change you suggested. Give me a sec, I'll revert back to the feature-gated version.

[Lokathor]

Using older than the current stable rust is kinda "theoretical", so it's fine either way.