formatter won't keep multiline for exceptions

[tx46]

how to test:

1. type type t = {name: string, msg: string} and run formatter
2. notice still single line
3. type

type t = {
  name: string,
  msg: string,
}

and run formattter
4. notice still four lines
5. type exception Foo({name: string,msg: string}) and run formatter
6. notice still single line
7. type

exception Foo({
  name: string,
  msg: string,
})

and run formatter
8. BUG: formatter forces it to single line