Ch6, and Ch18 should be merged

[Zexbe]

These two sections have a lot of things in common:
Enums and pattern matching: https://doc.rust-lang.org/book/ch06-00-enums.html
Patterns and Matching: https://doc.rust-lang.org/book/ch18-00-patterns.html

I feel it would be a lot more valuable to learn them at the same time by either merging them, or having them near each other. If they can't be merged, or put near each other, I think it would be useful if chapter 6 mentioned that more details could be found about patterns in chapter 18.

[carols10cents]

We're not going to be doing any large reorganizations of the book at this time, but I would be fine adding a link from the section in chapter 6 on match to chapter 18.