Add rbind argument to build_datalist()

[Thomas J. Leeper]

• An rbind = FALSE default argument would return the current data structure. rbind = TRUE would pass that through do.call("rbind", x) to return a single data frame.
• Add this to calls within prediction() methods. That should improve performance by eliminating the for-loop.

(But, unfortunately, for calculating variances in margins::margins(), we still need to loop over the list.)