User friendly enhancements to == in filter()

[hadley]

These are common problems (particularly amongst newer users), that would be very nice to hint about:

blah <- 5
filter(df, blah == blah)
#> Warning: You did `x == x` which always equals TRUE.

filter(df, blah == c("x", "y"))
#> Warning: the right hand side of == has multiple values.
#> Did you mean blah %in% c("x", "y")?
filter(df, blah != c("x", "y"))

I think we could implement by providing a custom == override in the scope.

[romainfrancois]

Do we want this for 0.8.0. This might be related to #3919 but could also be done independently if we want this right now.

[hadley]

I don't think we need to push this to the next release.

[hadley]

Now tracking in tidyverse/funs#33