Lazy deserialization

[muzarski]

Currently, rows result are eagerly deserialized to Vec<Row>. We can definitely improve that.

1. We do not need to store Vec<CassRow> in CassRowsResult. This because there is no API function that returns a row at given index. We only need to deserialize the first row (cass_result_first_row) and store it in CassRowsResult.
2. We do not need to (and should not) deserialize values to CqlValue. Because of this, we are not able to implement cass_value_get_bytes, which should return serialized value. We should rather deserialize to some partially-serialized type, which contains the bytes and information about the CQL column type.

### Tasks
- [ ] #50 
- [ ] https://github.com/scylladb/cpp-rust-driver/pull/213