11598: feat: Parse destructuring assignment r=Veykril a=ChayimFriedman2
Part of #11532.
Lowering is not as easy and may not even be feasible right now as it requires generating identifiers: `(a, b) = (b, a)` is desugared into
```rust
{
let (<gensym_a>, <gensym_b>) = (b, a);
a = <gensym_a>;
b = <gensym_b>;
}
```
rustc uses hygiene to implement that, but we don't support hygiene yet.
However, I think parsing was the main problem as lowering will just affect type inference, and while `{unknown}` is not nice it's much better than a syntax error.
I'm still looking for the best way to do lowering, though.
Fixes#11454.
Co-authored-by: Chayim Refael Friedman <chayimfr@gmail.com>
8f504dc873