Array indexing doesn't work


```
>>> sarray = Table('sarray', m, autoload_with=engine)
>>> print(select(sarray.c.s[0]).compile(engine))
Traceback (most recent call last):
...
sqlalchemy.exc.UnsupportedCompilationError: Compiler <sqlalchemy_bigquery.base.BigQueryCompiler object at 0x7fbb4075adc0> can't render element of type <built-in function getitem> (Background on this error at: https://sqlalche.me/e/14/l7de)
```