export `readr:::collectorGuess`

[njtierney]

Hi Hadley,

As discussed in the visdat issue, it would be great if you could export readr:::collectorGuess.

Thanks again for your help!

[hadley]

Or better collector_guess()

[njtierney]

Heck yes! Who doesn't love snake_case.

[njtierney]

Thank you!