Put back feature flags

Author: nhynes

rust/common/src/lib.rs

@@ -20,7 +20,7 @@
 //! This crate contains the refactored basic components required
 //! for `runtime` and `frontend` TVM crates.
 
-#![feature(box_syntax, trait_alias)]
+#![feature(box_syntax, trait_alias, type_alias_enum_variants)]
 
 #[macro_use]
 extern crate failure;

rust/frontend/src/lib.rs

@@ -30,7 +30,7 @@
 //!
 //! Checkout the `examples` repository for more details.
 
-#![feature(box_syntax)]
+#![feature(box_syntax, type_alias_enum_variants)]
 
 #[macro_use]
 extern crate failure;