Implement tbl_sum.tbl_sql()

[krlmlr]

Currently, dbplyr implements print.tbl_lazy() and print.tbl_sql(), but tbl_sql objects inherit from tbl_lazy. If we remove print.tbl_sql(), printing is handled by print.tbl_lazy(), which will print only the query plan but not the data.

@hadley: Okay to get rid of print.tbl_lazy(), and implement a different method (e.g., render()) to show the query plan of a lazy object?

[hadley]

Yeah, that's fine. The function to show the query plan doesn't need to be exported for now.