Update structure of internals with all the stacking...so much stacking

[ddsjoberg]

At the moment, when both soc and ae are specified, each section of SOC is 2 separate tbl_summary() tbls: one for the SOC and one for the AEs that are then stacked. THEN each SOC group is then stacked. This is a lot of duplicated work within the many calls to tbl_summary()

I think it would be faster if we built one tbl_summary() for all the AEs and one tbl_summary() for all the SOCs. We could then split the tables by SOC and make a single tbl_stack() for all the tbls.

@shannonpileggi is it ever possible that an AE term can appear within two or more SOCs? This would mess up the merging of the SOC terms and this approach would not work.