Generalize complex Cassandra types

[henrikfoss]

Currently simple types (int, float etc.), list and dicts are supported. And in pr #424 I added support for list of dicts.

But what if I wanted to make a list of lists that contains dicts or something even bigger? In that case each new complex type would require some messy code to implement. It would be nice if we can come up with a generalized solution for these kinds.