Incorrect conversion of custom fat arrow operator

[jwhitley]

refmt doesn't convert uses of an OCaml custom fat arrow operator let (=>) ... to Reason sensibly.

Please see reasonml/reason-cli#98 for details. I'm referencing that issue here, since reason-tools apparently keeps a copy of the refmt code locally in this repo, and this is where I first encountered this bug. Feel free to close if this is simply redundant.