Speed up add_overall()

[ddsjoberg]

The add_overall() function is pretty slow. It's slow because I am re-running the slowest parts of the function multiple times: calculating the complete data.

But this can be updated by updating the complete data, rather than re-calculating it each time. This will take a major refactoring of .complete_ae_data(), internals of all tbl_ae*() functions, and of add_overall().

This is a low priority.