"Convert `match` to let-else" assist

[jonas-schievink]

(let-else will be stable in about 6 weeks from now (Rust 1.65); we should probably not ship this assist before then)

It would be nice to have an assist to rewrite let statements with a match like the following, to their corresponding let-else form:

let val = match opt {
    Some(it) => it,
    None => return,
};

->

let Some(val) = opt else { return };

Criteria for the assist should be:

• let statement whose initializer is a match expression
• match has 2 arms, the first extracts data, the second has a diverging expression

[matklad]

We should also adjust existing “convert to early return” assist to use the new syntax

[jhgg]

match has 2 arms, the first extracts data, the second has a diverging expression

Probably should handle:

let val = match opt {
    None => return,
    Some(it) => it,
};

As well.

[lnicola]

https://user-images.githubusercontent.com/308347/200244795-545f0465-ec0f-4d8c-8f70-5f08aea99bf0.mp4

[fmease]

Ah, I just noticed this issue. It's a duplicate / subset? of #11908.