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/libstd/net/ip.rs

@@ -359,7 +359,7 @@ impl fmt::Display for Ipv6Addr {
                     let mut cur_span_len = 0;
                     let mut cur_span_at = 0;
 
-                    for i in range(0, 8) {
+                    for i in 0..8 {
                         if segments[i] == 0 {
                             if cur_span_len == 0 {
                                 cur_span_at = i;