Feature request: rename `fct_na_value_to_level` etc to `fct_na_to_level` etc as more readable

[davidhodge931]

Great work on the new functions converting NAs to levels/values!

This is subjective, but I wonder if the functions would be better named in a more concise way for better human readability. As NA can only either be a value or a level, it makes sense to me that you don't need to specify what it is being changed from

So status quo:
fct_na_value_to_level
fct_na_level_to_value

Proposed:
fct_na_to_level
fct_na_to_value

[hadley]

Hmmm, interesting idea. Will definitely contemplate it next time I'm working on forcats!