Export convert_to_scalar

One of the ideas for pgadmin for the future will be a statistics table
visualizer which amongst other things could chart the histograms so people can
see if they represent the distribution of data they expect.

To do this pgadmin would need access to convert_to_scalar so it understands
where to plot values from the histogram (and because where to place them is
the only way to tell how tall to make the bars).

As the comments indicate convert_to_scalar is a bit of a hack and we may want
to reimplement it some day but there's no particular need to do so for this
task.

There are two options here, we can either provide a user-accessible catalog
function to access convert_to_scalar in pg_proc, or we can just export
convert_to_scalar and allow pgadmin's adminpack to define the corresponding
function.

I've actually already done both so I could play with it, I can post whichever
people prefer or both to patches. It would be good to get one or the other
done in 8.3 so that if I or anyone else implements the charting with for
pgadmin they can support 8.3 instead of having to wait until 8.4.

--
Gregory Stark
EnterpriseDB http://www.enterprisedb.com