Export user visible function to make use of convert_to_scalar

Attached is a patch which implements, as discussed briefly on -hackers, a
user-visible function to get at the information that convert_to_scalar uses to
generate selectivity estimates.

The main use case for this is for tools such as pgadmin which want to make
sense of the histograms, they need to know where and how tall to draw each
region of the histogram.

I didn't add any documentation because I couldn't find a reasonable place to
put such a low level function. I don't think we would want to encourage
applications to use it unless they're trying to work with statistics tables
anyways so documenting it on the comparison operators page -- the only page
where it seems it would make sense -- would be a bad idea.