make `vis_guess` fast

Currently vis_guess is a bit slow and it'd be nice to make it faster.

We found that using vapply was much faster than using purrr, which is a bit of a shame as I think that the current code doesn't read as nice as purrr code (dare I say it, it doesn't purrr).

The way I see it we have two options to really speed things up.

1. Write some C++ code based on the readr::::CollectorGuess code, so that it deals with a vector, rather than a single element.
2. Use one of the functions from the parallel library, like mcapply.