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/core/src/lib.rs

@@ -254,6 +254,8 @@ pub mod panicking;
 pub mod pin;
 pub mod raw;
 pub mod result;
+#[unstable(feature = "async_stream", issue = "79024")]
+pub mod stream;
 pub mod sync;
 
 pub mod fmt;