add flat_map_iter to maybe_rayon & feature-gate it

maybe_rayon/src/lib.rs

@@ -1,6 +1,6 @@
 #[cfg(not(feature = "parallel"))]
 use std::{
-    iter::{IntoIterator, Iterator},
+    iter::{IntoIterator, Iterator, FlatMap},
     slice::{Chunks, ChunksExact, ChunksExactMut, ChunksMut},
 };
 
@@ -223,13 +223,22 @@ impl<T: Send> MaybeParChunksMut<T> for [T] {
     }
 }
 
+#[cfg(not(feature = "parallel"))]
 pub trait ParallelIteratorMock {
     type Item;
     fn find_any<P>(self, predicate: P) -> Option<Self::Item>
     where
         P: Fn(&Self::Item) -> bool + Sync + Send;
+
+    fn flat_map_iter<U, F>(self, map_op: F) -> FlatMap<Self, U, F>
+    where
+        Self: Sized,
+        U: IntoIterator,
+        F: Fn(Self::Item) -> U;
+
 }
 
+#[cfg(not(feature = "parallel"))]
 impl<T: Iterator> ParallelIteratorMock for T {
     type Item = T::Item;
 
@@ -239,6 +248,15 @@ impl<T: Iterator> ParallelIteratorMock for T {
     {
         self.find(predicate)
     }
+
+    fn flat_map_iter<U, F>(self, map_op: F) -> FlatMap<Self, U, F>
+    where
+        Self: Sized,
+        U: IntoIterator,
+        F: Fn(Self::Item) -> U
+    {
+        self.flat_map(map_op)
+    }
 }
 
 #[cfg(feature = "parallel")]