Refactor and abstract rendering with render-pre-wrap

[bhagany]

This is a subset of the changes in #109 that seemed to get the most positive response. It's almost purely a refactor intended to make rendering more pluggable. The only new feature is the addition of tracing (aka :io.perun/trace metadata) to render and collection.

[podviaznikov]

Need to test this locally myself first. Give me a few days.
The only braking change here is the removal of :groupBy option from the collection task. Everything else should work the same way, is it correct?

[bhagany]

Take your time, I know I'm causing a lot of work :). I actually left the :groupby option in this branch, I think it should stay unless we add assortment, and so everything should work the same as before. The default :groupby is handled in the task instead of the +collection-defaults+ map, in order to handle passing :meta to render-pre-wrap. That deletion may have made it look like I removed :groupby.

[podviaznikov]

@bhagany I merged some other PRs and they cause conflict here. Can you please resolve it when you have time?

I tested all other PRs and they work for me quite nice. I really like :io.perun/trace

[bhagany]

Okay, it's resolved. Thanks!