added Random.join, implemented Random.bind via Random.join

Author: TysonMN

src/Hedgehog/Random.fs

@@ -41,14 +41,16 @@ module Random =
             |> unsafeRun seed size
             |> f)
 
-    let bind (f: 'a -> Random<'b>) (r: Random<'a>) : Random<'b> =
+    let join (r: Random<Random<'a>>) : Random<'a> =
         Random (fun seed size ->
             let seed1, seed2 = Seed.split seed
             r
             |> unsafeRun seed1 size
-            |> f
             |> unsafeRun seed2 size)
 
+    let bind (f: 'a -> Random<'b>) (r: Random<'a>) : Random<'b> =
+        r |> map f |> join
+
     let replicate (times: int) (r: Random<'a>) : Random<List<'a>> =
         Random (fun seed0 size ->
             let rec loop seed k acc =