Various API improvements.

 - Remove Connection.insert and select and move those up to the higher level classes
 - Implement Table.insert (with proper string escaping) and Table.truncate
Start to refactor/extend result handling.

 - CassandraResult and PreparedStatement are now structs instead of classes
 - Replaced CassandraResult.rows by a range interface to read each row separately
 - Rows can be read into statically typed columns instead of buffering the raw data in memory
 - Still missing support/untested: list/set/map/custom/counter