Remove extern crate
 - it became unnecessary after rust compiler update

codegen/src/lib.rs

diff --git a/codegen/src/lib.rs b/codegen/src/lib.rs
index e13fc97..49b1753 100644
--- a/codegen/src/lib.rs
+++ b/codegen/src/lib.rs
@@ -1,6 +1,4 @@
 #![recursion_limit = "128"]
-extern crate proc_macro;
-extern crate proc_macro2;
 
 use proc_macro::TokenStream;