Add ide-assists: merge let statements

[A4-Tacks]

Close #10651

Assist: merge_let_stmts

Merge multiple lets into a let tuple pattern, allowing for the use of multiple identical let-else.

fn main() {
    $0let a = 2;
    let b = 3;$0
}

->

fn main() {
    let (a, b) = (2, 3);
}

[Veykril]

I am not sure how useful this assist really is. It seems very niche, but given its self-contained (and thus could easily enough be removed again if we decide to) it's probably fine to merge for now. Thoughs? @rust-lang/rust-analyzer

[Veykril]

Thinking about this a bit more, I really don't see the usefulness in this. I am not quite sure why you'd want to turn multiple let statements into a single one that just does destructuring on the left, and merging via a tuple expression on the right. That seems like a net readability downgrade to me.

[A4-Tacks]

For related simple assignments, in single let statement may be more readable

Such as let (x, y) = (10, 5);