Improve Expect.equal's diff display

When viewing a diff, I frequently forget which one is the actual value and which the expected one.

There are two similar alternatives that [are popular](https://github.com/elm-explorations/test/issues/38#issuecomment-428258808) that both use the pipe operator in the display and that are both compatible with the reasoning that the current output is based on:
```
"first value"
╷
│ |> Expect.equal
╵
"second value"
```
```
"first value"
|> Expect.equal
"second value"
```

In order to play around with alternatives, check out [the Ellie](https://ellie-app.com/4hDgCpfQz6ja1).