Originally we tried to maintain the invariant that `{}` always match.
That is, that in the parse tree the pair of corresponding `{}` is always
first and last tokens of some nodes.
We had the code to validate that, but apparently it's been broken for
**years** since we introduced tokens/nodes split. Fixing it now makes
some tests fail.
It's unclear if we want to keep this invariant: there's a strong
motivation for breaking it in the following case:
```
use std::{ // unclosed paren
fn main() {
}
} // don't actually want to pair up this with the one from `use`
```
So let's fix the code, but disable it for the time being
defe805fb7