Jep 7: Filter expressions

This implements jep-7:

```
foo[?bar==`baz`]
```

This includes:

* filter expressions
* literal expressions
* modified subexpression grammar

See the `spec-jep-7` branch for the spec.

This also includes:

* syntax.json: to validate syntax errors
* abnfstress: to make it easier to test the abnf grammar


cc @mtdowling