It now reports the last inserted ID and affected row count for all statements,
not just the last one. This is useful to execute batches of statements such as
UPDATE with minimal roundtrips.

The approach taken is to track last insert id and affected rows using []int64
instead of a int64. Both are set in `mysqlResult`, and a new `mysql.Result`
interface makes them accessible to callers calling `Exec()` via `sql.Conn.Raw`.
For example:

```
conn.Raw(func(conn interface{}) error {
        ex := conn.(driver.Execer)
        res, err := ex.Exec(`
        UPDATE point SET x = 1 WHERE y = 2;
        UPDATE point SET x = 2 WHERE y = 3;
        `, nil)
        // Both slices have 2 elements.
        log.Print(res.(mysql.Result).AllRowsAffected())
        log.Print(res.(mysql.Result).AllLastInsertIds())
      })
```