support for unnest with multiple args

```sql
select unnest(ARRAY[1, 2], ARRAY[10, 20]);
ERROR: function unnest(integer[], integer[]) does not exist
```

This is used by Django on a standard migration path, so it's high priority. Django uses it in an insert like this:

```sql
INSERT INTO bug11 (a, b) SELECT * FROM UNNEST(ARRAY[1, 2], ARRAY[10, 20]);
```

Start by reproducing the two SQL examples in DoltgreSQL, then trace the unnest function handling for calls with multiple array arguments. Done means both SELECT unnest(...) and the Django-style INSERT ... SELECT * FROM UNNEST(...) execute successfully and return paired rows.

Written by the indexing model from the issue text.