Create new rustc_smir struct to map future crates

+ Add some information to the README.md

compiler/rustc_smir/src/lib.rs

@@ -11,5 +11,9 @@
     test(attr(allow(unused_variables), deny(warnings)))
 )]
 #![cfg_attr(not(feature = "default"), feature(rustc_private))]
-#![deny(rustc::untranslatable_diagnostic)]
-#![deny(rustc::diagnostic_outside_of_impl)]
+
+pub mod rustc_internal;
+pub mod stable_mir;
+
+// Make this module private for now since external users should not call these directly.
+mod rustc_smir;