Fix #642 Bad performance/memory leak for long list literals
Fix #642 Bad performance/memory leak for long list literals

A one-character fix that speeds up the formatting of long literals
and drastically reduces memory usage.

A very simple example is

```elm
module Small exposing (x)

x =
    [ 1, 2, 3
     , 4]
```

But with a list literal of many thousands of elements. The newline
forces all elements to get their own line. This is implemented as a
fold, but in practice this would often perform

```
longList ++ shortList
```

By reversing the fold (from `foldl` to `foldr`) that turns into

```
shortList ++ longList
```

Which requires less traversals and memory.

avh4/elm-format,avh4/elm-format,avh4/elm-format,avh4/elm-format