This PR auto-assign ids for anonymous variables straight into parser.
Otherwise - it's pretty easy to mess up with traversal order in the core
code and assign ids incorrectly.
For example, before the fix, following code worked incorrectly because
parameter values were assigned first to conflict clause instead of
values:
```rs
let mut stmt = conn.prepare("INSERT INTO test VALUES (?, ?), (?, ?) ON CONFLICT DO UPDATE SET v = ?")?;
stmt.bind_at(1.try_into()?, Value::Integer(1));
stmt.bind_at(2.try_into()?, Value::Integer(20));
stmt.bind_at(3.try_into()?, Value::Integer(3));
stmt.bind_at(4.try_into()?, Value::Integer(40));
stmt.bind_at(5.try_into()?, Value::Integer(66));
```
Closes#3455
81e7c26f55