This is an alternative to reasonml#1826 implementing just the brace preservation
code, to make it easier to be reviewed.

I think this is now in a better place to get into the codebase given
that we have `Reason_attributes` outside of `Reason_pprint_ast` and that
we now remove our own attributes before printing to OCaml.

I think we can probably refactor this code further in future PRs, as
well as add other stylistic preservation changes.