add anchors for chapter 6

[ntwilson]

I struggled a lot with this one. Mdbook is very picky about indentation, and there seems to be some edge cases that weren't well thought out when it comes to indented code blocks with anchors. In particular, it seems Mdbook just pastes in the code directly, including whitespace, and doesn't indent further when the code block was indented. So if I had an anchor like

instance Eq (NonEmpty a) where
  eq (NonEmpty x xs) (NonEmpty y ys) = x == y && xs == ys

and I used that in a code block like

## Exercises
1. In the the code below:
    ```haskell
    {{#include ../exercises/chapter6/test/no-peeking/Solutions.purs:eqNonEmpty}}
    ```
    we ...

that would get translated into

## Exercises
1. In the code below:
    ```haskell
    instance Eq (NonEmpty a) where
  eq (NonEmpty x xs) (NonEmpty y ys) = x == y && xs == ys
    ```
    we ...

which really throws off the document flow. The only three solutions I could come up with were

1. standardize the text indentation on size 4, and indent the source code far enough to get around this problem (6). So the original code block becomes

   instance Eq (NonEmpty a) where
         eq (NonEmpty x xs) (NonEmpty y ys) = x == y && xs == ys

2. make an anchor for each individual line of code, so we can indent each line as much as we need to in the .md text
3. don't use anchors at all

I opted for approach 1, but it feels really hacky, and I would welcome feedback on if any other approach should be taken.

[ntwilson]

Feel free to nix my extra commentary. It seemed to me that immediately dispelling any preconceived notions about the word "class" might be helpful

[ntwilson]

Again, feel free to nix my extra commentary. I thought it might confuse some readers why in every chapter up to this point, they've been given all the code they need already and can just start coding up the exercises, but in this chapter they suddenly have to copy+paste a bunch of types into their code.

[milesfrain]

Wow. That's some pretty annoying mdbook behavior. Opened rust-lang/mdBook#1626 to see if we can find a resolution. This might require making a PR to that project.