Add core::stream::Stream

This patch adds the `core::stream` submodule and implements `core::stream::Stream` in accordance with RFC2996.

Add feedback from @camelid

library/std/src/lib.rs

@@ -224,6 +224,7 @@
 #![feature(allocator_internals)]
 #![feature(allow_internal_unsafe)]
 #![feature(allow_internal_unstable)]
+#![feature(async_stream)]
 #![feature(arbitrary_self_types)]
 #![feature(array_error_internals)]
 #![feature(asm)]
@@ -448,6 +449,8 @@ pub use core::ptr;
 pub use core::raw;
 #[stable(feature = "rust1", since = "1.0.0")]
 pub use core::result;
+#[unstable(feature = "async_stream", issue = "79024")]
+pub use core::stream;
 #[stable(feature = "i128", since = "1.26.0")]
 #[allow(deprecated, deprecated_in_future)]
 pub use core::u128;