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/env.rs

@@ -18,7 +18,6 @@
 
 use prelude::v1::*;
 
-use iter::IntoIterator;
 use error::Error;
 use ffi::{OsStr, OsString};
 use fmt;