clean_by() takes a valid by input of a character vector (named or unnamed) and transforms
it into a list. Similar to common_by but does not require the actual data frames.
clean_by(by)A character vector of variables to join by. Same specification as used in
mutate-joins.
To join by different variables on x and y use a named vector.
For example, by = c("a" = "b") will match x$a to y$b.