This implements two simple SQL functions, building different types of
histograms from a t-digest. Equi-width histograms are the "usual" type
with each bin covering about 1/N of the domain, while equi-height
histograms have bins representing the same fraction of the data.

The functions are fairly trivial SQL functions, calculating boundaries
on either the domain or frequency, and then computing the other values
using the t-digest.

I'm not sure how accurate the histograms are - considering how t-digests
work, the bins at the ends should be the fine while the bins in the
middle could be somewhat less accurate.

One thing this made me realize is the extension only provides aggregate
functions, so when we need to work with t-digests in non-aggregate
contexts it's necessary to do unnecessary groupings etc. Might be handy
to provide some non-aggregate functions to make that unnecessary.