Rename and export probe_colwise_names

[jlegewie]

It is easy to support other classes with methods for mutate_, summarise_, mutate_at etc. *_if() variants, however, do not work with databases and we are unable to directly support it in spatial objects in spdplyr. Here are some thoughts from @lionel- :

The easy fix is to make probe_colwise_names() an exported generic. I'm not sure we want to abstract out function mapping in the dplyr interface though. So I don't know what can be done here.

Maybe have a generic map in purrr. I think we need a s3map() family in purrr. But until purrr depends on dplyr (there are plans to decouple) we can't have dplyr depend on purrr anyway.

mdsumner/spdplyr#10 (comment)

[hadley]

Lets make probe_colwise_names() generic. We'll just need to come up with a slighter shorter name. It might be reasonable for the default method to call collect() or as.data.frame() (for data bases, n will need be set to a small number), since I think most often the predicates are used for type information.