wip: attempt at not using slow shapeless for (large) co-products

[cvogt]

this is work in progress. it currently still leads to

[error] /Users/chris/xdotai/diff/test/Main.scala:25: could not find implicit value for parameter e: ai.x.diff.DiffShow[Foo]
[error]   implicitly[DiffShow[Parent]]
[error]             ^