The keyword `label` can't be used as a binding name :(

```
jq -n '. as $label | $label'
jq: error: syntax error, unexpected label, expecting IDENT (Unix shell quoting issues?) at <top-level>, line 1:
. as $label | $label
jq: 1 compile error
```

We should make sure all keywords can be used as `$keyword` bindings.