Mixins columns no longer work in 2.X

[jalaziz]

We have a use case where we place shared columns in traits and then inherit them when creating the table.

The TableHelper macro currently looks for the first table with columns, but doesn't check for partition keys. I understand that this was done to support inheritance. However, mixing in columns can be very useful when multiple tables all share the same columns.

An example can be found here: https://gist.github.com/jalaziz/f7f5030172acfccafb46a5d239975924

When defining tables with mixins, I find that compilation fails with could not find implicit value for parameter helper: com.outworkers.phantom.macros.TableHelpe....

[alexflav23]

@jalaziz I think we need to work on formalising all the requirements for inheritance, and right now we are willing to champion those efforts. Before we explicitly marked this is a "not supported", and perhaps not make that explicit in the code, but I think right now there are quite a few people requesting this feature.

I'm going to open a separate issue with a proposal, it's reasonably straight forward, but just in case, so everyone can contribute and we can reach some kind of consensus on what this should look like.

[alexflav23]

I'm going to centralise this in #770, as there are many related tickets that have inheritance support as a root cause.