Columnar derived types

[Protryon]

At the moment we have klickhouse::Row derivable trait and klickhouse::Block raw columnar storage. It would be ideal to have a derivable, easy-to-use klickhouse::Columnar derivable trait akin to klickhouse::Row but with the superior performance of actually being columnar.

No idea when I'll have time/motivation to implement this, but it's been on my mind.