improve extra::stats

[graydon]

The extra::stats module is a nice start but could use some improvements. I have a wip branch I mean to land on this. It has (or should have, when done):

• Actual test vectors generated from R, which I trust a bit.
• Docs.
• Support for percentiles (and helpers for quartiles, 5- and 7-value summaries, IQRs)
• Code for ASCII-art box-and-whisker plots
• A Summary structure distinct from the sample input

This is prelude work for #6810

[catamorphism]

I didn't think we were using the 0.8 milestone for anything. Nominating for milestone 4, well-covered.

[huonw]

It looks like all of that list is done, closing. Feel free to reopen if someone wishes to co-opt this bug for other improvements.