`enframe()` also works for data frames, creates a packed data frame

[krlmlr]

Requires https://github.com/rubenarslan/codebook/issues/68.

[rubenarslan]

Sorry for the ridiculously long delay. But yes, my tests pass with this version. If I was enframing data frames there, then I was not checking my inputs well enough, but I can't reproduce the problem now.