Remove deprecated Range::step_by (use Iterator::step_by instead)

Follow-up to rust-lang#41439 (comment)

While doing so, remove the now-unused `step`, `steps_between`, and `is_negative` methods from `Step`.

Mostly simple, but needed two interesting changes:
* Override `Iterator::size_hint` for `iter::StepBy` (so hints aren't lost)
* Override `Iterator::size_hint` for `ops::RangeFrom` (so `(0..).size_hint()` returns what `(0..).step_by(1).size_hint()` used to)
(It turns out that `(0..).step_by(d)` is used in a bunch of tests, from `cycle` to `vec_deque`.)

Incidentally fixes rust-lang#41477

src/doc/unstable-book/src/SUMMARY.md

@@ -199,7 +199,6 @@
     - [sort_internals](library-features/sort-internals.md)
     - [sort_unstable](library-features/sort-unstable.md)
     - [splice](library-features/splice.md)
-    - [step_by](library-features/step-by.md)
     - [step_trait](library-features/step-trait.md)
     - [str_checked_slicing](library-features/str-checked-slicing.md)
     - [str_escape](library-features/str-escape.md)

src/libcollections/tests/lib.rs

@@ -21,7 +21,7 @@
 #![feature(placement_in_syntax)]
 #![feature(rand)]
 #![feature(splice)]
-#![feature(step_by)]
+#![feature(iterator_step_by)]
 #![feature(str_escape)]
 #![feature(test)]
 #![feature(unboxed_closures)]

src/libcore/iter/iterator.rs

@@ -130,9 +130,10 @@ pub trait Iterator {
     ///
     /// ```
     /// // an infinite iterator has no upper bound
+    /// // and the maximum possible lower bound
     /// let iter = 0..;
     ///
-    /// assert_eq!((0, None), iter.size_hint());
+    /// assert_eq!((usize::max_value(), None), iter.size_hint());
     /// ```
     #[inline]
     #[stable(feature = "rust1", since = "1.0.0")]

src/libcore/iter/mod.rs

@@ -311,9 +311,6 @@ pub use self::iterator::Iterator;
            reason = "likely to be replaced by finer-grained traits",
            issue = "27741")]
 pub use self::range::Step;
-#[unstable(feature = "step_by", reason = "recent addition",
-           issue = "27741")]
-pub use self::range::StepBy as DeprecatedStepBy;
 
 #[stable(feature = "rust1", since = "1.0.0")]
 pub use self::sources::{Repeat, repeat};
@@ -553,6 +550,19 @@ impl<I> Iterator for StepBy<I> where I: Iterator {
             self.iter.nth(self.step)
         }
     }
+
+    #[inline]
+    fn size_hint(&self) -> (usize, Option<usize>) {
+        let inner_hint = self.iter.size_hint();
+
+        if self.first_take {
+            let f = |n| if n == 0 { 0 } else { 1 + (n-1)/(self.step+1) };
+            (f(inner_hint.0), inner_hint.1.map(f))
+        } else {
+            let f = |n| n / (self.step+1);
+            (f(inner_hint.0), inner_hint.1.map(f))
+        }
+    }
 }
 
 /// An iterator that strings two iterators together.