trappy/ftrace: merge primary and secondary DFs based on pivot

Open Darryl Green requested to merge github/fork/joelagnel/merged-dfs into master

Created by: joelagnel

I hacked something up that's working, just posting here. Basic idea is concatenate the DFS, mark them as primary/secondary, walk through them and remember the secondary information, propagate it to primary and rebuild the primary DF. I haven't written test cases yet. (CC @derkling @bjackman @sinkap)

Here's a gist showing it working in @derkling 's notebook:

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