Merge #1733

Merge #1733

1733: Parse arbitrarily complex `box` patterns. r=matklad a=ecstatic-morse

This fully resolves the pattern part of #1412 by enabling the parsing of complex `box` patterns such as:

```rust
let box Struct { box i, j: box Inner(box &x) } = todo!();
```

This introduces a new `ast::BoxPat` (in the mold of `ast::RefPat`) that gets translated to `hir::Pat::Missing`.

Co-authored-by: Dylan MacKenzie <ecstaticmorse@gmail.com>