Implement Summary Functions in Data-Frame

Nobody has claimed this yet.

This issue also appears in lisp-stat/data-frame#4, repeated and expanded here because the lisp-stat repo is the 'landing point' for most people.

Lisp-Stat version 1.0 had a `summary` function that did basic formatting of a data frame. When designing version 2 of data-frames we looked at how R implemented summary functions and found that there were several packages for summarizing a data frame. There's an [article on a few of them](https://dabblingwithdata.wordpress.com/2018/01/02/my-favourite-r-package-for-summarising-data/). When we looked at how the Tidy guys did it, the consensus was (basically), 'there isn't a summary that will make everyone happy' (see the thread [A better summary function](https://github.com/tidyverse/dplyr/issues/1514))

So what we decided to do in data frame 2.0 is not try to write a fancy summary function. Instead, we focused on improving the underlying summary data structures so that it would be easier to build things like [skimr ](https://ropensci.org/blog/2017/07/11/skimr/) or [summarytools](https://cran.r-project.org/web/packages/summarytools/vignettes/introduction.html).

This issue serves as a suggestion to anyone that would like to take up the task of creating a nice summary package along the lines of the ones available for R. This will be a good way to learn the basics of data-frame, and a good way to learn common lisp, if you don't already know it.

Start by reading the related lisp-stat/data-frame#4 issue and the linked R summary-package examples. The issue names no repository files, tests, or entry points, so the scope and expected behavior need to be defined before implementation; done means an agreed summary package for data frames.

Written by the indexing model from the issue text.