Rollup merge of #6402 - camsteffen:collapsible-match, r=llogiq

Add Collapsible match lint

changelog: Add collapsible_match lint

Closes #1252
Closes #2521

This lint finds nested `match` or `if let` patterns that can be squashed together. It is designed to be very conservative to only find cases where merging the patterns would most likely reduce cognitive complexity.

Example:

```rust
match result {
    Ok(opt) => match opt {
        Some(x) => x,
        _ => return,
    }
    _ => return,
}
```
to
```rust
match result {
    Ok(Some(x)) => x,
    _ => return,
}
```

These criteria must be met for the lint to fire:

* The inner match has exactly 2 branches.
* Both the outer and inner match have a "wild" branch like `_ => ..`. There is a special case for `None => ..` to also be considered "wild-like".
* The contents of the wild branches are identical.
* The binding which "links" the matches is never used elsewhere.

Thanks to the hir, `if let`'s are easily included with this lint since they are desugared into equivalent `match`'es.

I think this would fit into the style category, but I would also understand changing it to pedantic.

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