std: Deprecate the RandomAccessIterator trait

This trait has not proven itself over time as being core and fundamentally
useful to iterators, so it's being deprecated to allow time to iterate on it out
of tree.

src/libcore/iter.rs

@@ -1230,6 +1230,10 @@ impl<'a, I: DoubleEndedIterator + ?Sized> DoubleEndedIterator for &'a mut I {
 #[unstable(feature = "iter_idx",
            reason = "not widely used, may be better decomposed into Index \
                      and ExactSizeIterator")]
+#[deprecated(since = "1.2.0",
+             reason = "trait has not proven itself as a widely useful \
+                       abstraction for iterators, and more time may be needed \
+                       for iteration on the design")]
 pub trait RandomAccessIterator: Iterator {
     /// Returns the number of indexable elements. At most `std::usize::MAX`
     /// elements are indexable, even if the iterator represents a longer range.