Constant column, object columns and native columns need better interop

[Vindaar]

Currently we are a bit too quick to convert constant columns into object columns. In many cases a native column as an extension would certainly be enough, e.g. stacking two constant int columns on top of another should yield an int column and not an object column.

The whole column assignment procedures need a thorough set of tests + possibly some rewrite (there's duplication in there etc).

[Vindaar]

To a large extent this is addressed in #24.