This builds on #194, #245, & #257.

Allow the user to specify the exact col_types that are passed to readr::read_csv() inside redcap_read_oneshot().

Then readr won't have to guess what the data types should be. It (justifiably) guesses conservatively, so variables like record_id are floating-point numbers, instead of integers.

Thanks to @isaactpetersen and others for these discussions.