Remove code duplicating stdlib (#1239)

* Get rid of superfluous ReverseBlockRootIterator

* Get rid of superfluous ReverseStateRootIterator and ReverseChainIterator

* cargo fmt

beacon_node/store/src/iter.rs

@@ -294,52 +294,6 @@ fn slot_of_prev_restore_point<E: EthSpec>(current_slot: Slot) -> Slot {
     (current_slot - 1) / slots_per_historical_root * slots_per_historical_root
 }
 
-pub type ReverseBlockRootIterator<'a, E, S> =
-    ReverseHashAndSlotIterator<BlockRootsIterator<'a, E, S>>;
-pub type ReverseStateRootIterator<'a, E, S> =
-    ReverseHashAndSlotIterator<StateRootsIterator<'a, E, S>>;
-
-pub type ReverseHashAndSlotIterator<I> = ReverseChainIterator<(Hash256, Slot), I>;
-
-/// Provides a wrapper for an iterator that returns a given `T` before it starts returning results of
-/// the `Iterator`.
-pub struct ReverseChainIterator<T, I> {
-    first_value_used: bool,
-    first_value: T,
-    iter: I,
-}
-
-impl<T, I> ReverseChainIterator<T, I>
-where
-    T: Sized,
-    I: Iterator<Item = T> + Sized,
-{
-    pub fn new(first_value: T, iter: I) -> Self {
-        Self {
-            first_value_used: false,
-            first_value,
-            iter,
-        }
-    }
-}
-
-impl<T, I> Iterator for ReverseChainIterator<T, I>
-where
-    T: Clone,
-    I: Iterator<Item = T>,
-{
-    type Item = T;
-
-    fn next(&mut self) -> Option<Self::Item> {
-        if self.first_value_used {
-            self.iter.next()
-        } else {
-            self.first_value_used = true;
-            Some(self.first_value.clone())
-        }
-    }
-}
-
 #[cfg(test)]
 mod test {
     use super::*;