get group_by() working and interacting nicely with the other core dplyr verbs

[pgbovine]

these are the critical test cases to get working here: https://github.com/seankross/mario/tree/main/inst/test/pg-dump_2021-11-05/groupby-most-important

[pgbovine]

Make sure that it also works if the group_by was done in an earlier expression. e.g.,

mt <- mtcars %>% as_tibble() %>% select(mpg, cyl, hp) %>% group_by(cyl)

# we're doing this on a separate line
mt %>% summarize(Mean = mean(mpg))