feat fut: add infix, builds a INFIX as a 1st class module

src/fut.ml

@@ -354,3 +354,9 @@ end) =
 Infix_ (struct
   let pool = Some X.pool
 end)
+
+let[@inline] infix pool : (module INFIX) =
+  let module M = Infix (struct
+    let pool = pool
+  end) in
+  (module M)

src/fut.mli

@@ -154,3 +154,7 @@ include INFIX
 module Infix (_ : sig
   val pool : Pool.t
 end) : INFIX
+
+val infix : Pool.t -> (module INFIX)
+(** [infix pool] makes a new infix module.
+    @since 0.2 *)