std: Add Default/IntoIterator/ToOwned to the prelude

This is an implementation of [RFC 1030][rfc] which adds these traits to the
prelude and additionally removes all inherent `into_iter` methods on collections
in favor of the trait implementation (which is now accessible by default).

[rfc]: https://github.com/rust-lang/rfcs/pull/1030

This is technically a breaking change due to the prelude additions and removal
of inherent methods, but it is expected that essentially no code breaks in
practice.

[breaking-change]
Closes #24538

src/libstd/path.rs

@@ -103,7 +103,7 @@ use core::prelude::*;
 use ascii::*;
 use borrow::{Borrow, IntoCow, ToOwned, Cow};
 use cmp;
-use iter::{self, IntoIterator};
+use iter;
 use mem;
 use ops::{self, Deref};
 use string::String;