Allow arguments of derivers to have default values

[panglesd]

Deriving arguments are always extracted as options, or bool: see the two function creating arguments: arg and flag.

It could be nice to add a function to have an argument with a default value, to avoid handling the default case in the main function, as in let arg = match arg with Some v -> v | None -> default_value in (reported by @sim642, see ocaml-ppx/ppx_deriving#263 (comment)).

The added function would be:

val arg_default : 
  string ->
  (expression, 'a -> 'a, 'a) Ast_pattern.t ->
  default:'a ->
  'a param

[panglesd]

@Annosha is working on this one