Add alias for iterator fold

fold is known in python and javascript as reduce,
not sure about inject but it was written in doc there.

Author: pickfire

core/src/iter/traits/iterator.rs

@@ -1977,6 +1977,8 @@ pub trait Iterator {
     /// assert_eq!(result, result2);
     /// ```
     #[inline]
+    #[doc(alias = "reduce")]
+    #[doc(alias = "inject")]
     #[stable(feature = "rust1", since = "1.0.0")]
     fn fold<B, F>(mut self, init: B, mut f: F) -> B
     where