Deprecate range, range_step, count, distributions

This commit deprecates the `count`, `range` and `range_step` functions
in `iter`, in favor of range notation. To recover all existing
functionality, a new `step_by` adapter is provided directly on `ops::Range`
and `ops::RangeFrom`.

[breaking-change]

src/libcore/prelude.rs

@@ -29,6 +29,7 @@ pub use marker::{Copy, Send, Sized, Sync};
 pub use ops::{Drop, Fn, FnMut, FnOnce};
 
 // Reexported functions
+#[allow(deprecated)]
 pub use iter::range;
 pub use mem::drop;