Add content on Row Types and Variants

[milesfrain]

I'd like to see an explanation on how to use Prim.Row in the book.

Details on how to use Variant would also be nice. I think the existing readme could be expanded and either find a new home in the book or be linked to from the book.

Both of these related topics could fill a new chapter.

There's light coverage of rows in Chapter 5 and Chapter 9, and there used to be some other minor material in Chapter 8 with the older Eff.

Here's a list of articles on the topic, plus two more: 1 2. I'd like to distill some of this content and put in in the book so beginners don't have to do so much digging for it.

Tagging @drewolson, since this is another idea of a topic to cover.

[milesfrain]

Found more content in Jordan's Reference (TOC):

• Type Level Programming
  • Syntax
  • Rows
• Variant

[milesfrain]

As discussed in #25, it doesn't make much sense to restore the original "Record and Rows" section to Ch8. But that lost content can be incorporated into this new proposed section.