Strip prefixes in WalkEntrys.

This change makes stripped paths available to `FileIterator` filters in
both filtrate and residue by supporting a stripped path in `WalkEntry`s.

src/walk/filter.rs

@@ -142,23 +142,6 @@ impl<T, U> Feed for (T, U) {
     type Residue = U;
 }
 
-// TODO: Remove this?
-pub trait Isomorphic: Feed
-where
-    SeparationFiltrate<Self>: Isomer<SeparationResidue<Self>>,
-    for<'a> SeparationResidue<Self>:
-        'a + Isomer<SeparationFiltrate<Self>, Substituent<'a> = SeparationSubstituent<'a, Self>>,
-{
-}
-
-impl<F, R, S> Isomorphic for S
-where
-    F: Isomer<R>,
-    for<'a> R: 'a + Isomer<F, Substituent<'a> = <F as Isomer<R>>::Substituent<'a>>,
-    S: Feed<Filtrate = F, Residue = R>,
-{
-}
-
 pub trait Isomer<T>: Sized {
     type Substituent<'a>
     where
@@ -180,9 +163,7 @@ impl<S> Separation<S>
 where
     S: Feed,
 {
-    // TODO: Provide a trait-based and intentionally very limited way to implement a base
-    //       `Feed` that constructs a `Filtrate<T>` from unfiltered `Iterator` items of `T`.
-    pub(in crate::walk) fn from_inner_filtrate(filtrate: S::Filtrate) -> Self {
+    fn from_inner_filtrate(filtrate: S::Filtrate) -> Self {
         Separation::Filtrate(Filtrate::new(filtrate))
     }
 
@@ -276,7 +257,11 @@ where
         }
     }
 
-    pub fn filter_isomer_tree<I, F>(self, cancellation: WalkCancellation<'_, I>, f: F) -> Self
+    pub fn filter_tree_by_substituent<I, F>(
+        self,
+        cancellation: WalkCancellation<'_, I>,
+        f: F,
+    ) -> Self
     where
         U: 'static + From<S::Filtrate>,
         S::Filtrate: Isomer<TreeResidue<U>>,
@@ -339,21 +324,27 @@ where
     }
 }
 
-// TODO: Remove this?
-impl<S> Feed for Separation<S>
-where
-    S: Feed,
-{
-    type Filtrate = S::Filtrate;
-    type Residue = S::Residue;
-}
-
 pub trait SeparatingFilter {
     type Feed: Feed;
 
     fn feed(&mut self) -> Option<Separation<Self::Feed>>;
 }
 
+impl<I> SeparatingFilter for I
+where
+    I: SeparatingFilterInput,
+{
+    type Feed = <I as SeparatingFilterInput>::Feed;
+
+    fn feed(&mut self) -> Option<Separation<Self::Feed>> {
+        self.next().map(Separation::from_inner_filtrate)
+    }
+}
+
+pub trait SeparatingFilterInput: Iterator {
+    type Feed: Feed<Filtrate = Self::Item>;
+}
+
 pub trait SkipTree {
     fn skip_tree(&mut self);
 }
@@ -366,8 +357,9 @@ where
     I: SkipTree,
 {
     fn skip_tree(&mut self) {
-        // Client code is only able to move a `WalkCancellation` into `Filtered::filter_map_tree`,
-        // at which point the filtered item should be the current item of the iterator.
+        // Client code is only able to move a `WalkCancellation` into
+        // `Separation::filter_map_tree`, at which point the filtered item should be the current
+        // item of the iterator.
         self.0.skip_tree()
     }
 }