Quicktest Positions

`Positions` is tested to be fused, for specialized methods and has two doctests. No other test.
While I was adding `Enumerate` in `Positions` internals, running `cargo test positions` told me `rfold` was wrong. The silly mistake was on `next_back` instead and this test would have helped me.
Plus, I noticed that the documentation was just a bit wrong.

src/lib.rs

@@ -1806,7 +1806,7 @@ pub trait Itertools: Iterator {
     /// Return an iterator adaptor that yields the indices of all elements
     /// satisfying a predicate, counted from the start of the iterator.
     ///
-    /// Equivalent to `iter.enumerate().filter(|(_, v)| predicate(v)).map(|(i, _)| i)`.
+    /// Equivalent to `iter.enumerate().filter(|(_, v)| predicate(*v)).map(|(i, _)| i)`.
     ///
     /// ```
     /// use itertools::Itertools;

tests/quick.rs

@@ -604,6 +604,12 @@ quickcheck! {
         let b = &b[..len];
         itertools::equal(zip_eq(a, b), zip(a, b))
     }
+    fn equal_positions(a: Vec<i32>) -> bool {
+        let with_pos = a.iter().positions(|v| v % 2 == 0);
+        let without = a.iter().enumerate().filter(|(_, v)| *v % 2 == 0).map(|(i, _)| i);
+        itertools::equal(with_pos.clone(), without.clone())
+            && itertools::equal(with_pos.rev(), without.rev())
+    }
     fn size_zip_longest(a: Iter<i16, Exact>, b: Iter<i16, Exact>) -> bool {
         let filt = a.clone().dedup();
         let filt2 = b.clone().dedup();