group string_to string_of functions into record

[ygrek]

this will help consumers to write generic code where this pair of functions can be passed around (it is ofc possible to do now just more verbose and annoying)

[smondet]

Instead of a record why not a generate a submodule:

module Type_name = struct
  type t = type_name
  let of_string = type_name_of_string
  (* etc ...*)
end

• can still be used as a record (first class module)
  • in that case the modules are structurally typed so no need to share a
    record type with ATD libs from code using that feature
• can also be more easily used as a common functor argument