Implement na_if() about funs HOT 2 OPEN

We need to figure out the semantics — I think probably it should using matching not equality. And maybe it should only be vectorised in its first argument? Maybe it should just be this?

na_if <- function(x, na) {
 x[vec_in(x, na)] <- NA
 x
}

Which makes it a special case of case_in(): case_in(x, y ~ NA)

from funs.

Regarding na_if() being match-like, we document that na_if() is like SQL NULLIF(), and that uses equality. And a few people use it that way (after looking at GitHub). So it is probably best to leave it like this and promote some other way to do match-like replacement with NAs. See my full analysis in: tidyverse/dplyr#6329 (comment)

from funs.