Improve: tidying Fmt_ast (#808)
Improve: tidying Fmt_ast (#808)

- use str when possible
- deduplicate fmt_label_arg
- deduplicate fmt_args
- don't use @@ if it doesn't require extra parentheses

ocaml-ppx/ocamlformat,ocaml-ppx/ocamlformat,ocaml-ppx/ocamlformat