Use local substitution in signatures

- I find it easier to read
- This avoids the extra indentation created by ocamlformat.0.26.1

src/natural_ratio.mli

@@ -14,31 +14,30 @@ val multiply : t -> t -> t
 val divide : t -> t -> t
 
 module Reduced : sig
-    type natural_ratio
-
-    (** A [natural_ratio] kept in its primed decomposition form. *)
-    type t [@@deriving sexp_of]
-
-    val to_string : t -> string
-    val equal : t -> t -> bool
-    val one : t
-
-    (** [prime] is assumed to be a prime number. The resulting [t] will be invalid
-        and the behavior of the rest of the module unspecified if it isn't. The
-        exponent is expected to be non null. Checking that [prime] is indeed
-        prime is only done if its value is small enough. This should suffice in
-        practice, since creating such values is done with small ps. *)
-    val create_exn : prime:int -> exponent:int -> t
-
-    val compound : t list -> t
-    val inverse : t -> t
-    val multiply : t -> t -> t
-    val divide : t -> t -> t
-    val to_natural_ratio : t -> natural_ratio
-
-    (** When both the numerator and denominator and below a small bound, this
-        function can be used to instantiate a [t]. Raises if out of supported
-        bounds. *)
-    val of_small_natural_ratio_exn : numerator:int -> denominator:int -> t
-  end
-  with type natural_ratio := t
+  type natural_ratio := t
+
+  (** A [natural_ratio] kept in its primed decomposition form. *)
+  type t [@@deriving sexp_of]
+
+  val to_string : t -> string
+  val equal : t -> t -> bool
+  val one : t
+
+  (** [prime] is assumed to be a prime number. The resulting [t] will be invalid
+      and the behavior of the rest of the module unspecified if it isn't. The
+      exponent is expected to be non null. Checking that [prime] is indeed
+      prime is only done if its value is small enough. This should suffice in
+      practice, since creating such values is done with small ps. *)
+  val create_exn : prime:int -> exponent:int -> t
+
+  val compound : t list -> t
+  val inverse : t -> t
+  val multiply : t -> t -> t
+  val divide : t -> t -> t
+  val to_natural_ratio : t -> natural_ratio
+
+  (** When both the numerator and denominator and below a small bound, this
+      function can be used to instantiate a [t]. Raises if out of supported
+      bounds. *)
+  val of_small_natural_ratio_exn : numerator:int -> denominator:int -> t
+end