Remove packed_struct dependency; closes #1271 and #1284 (#1282)

cranelift/codegen/shared/src/lib.rs

@@ -20,10 +20,6 @@
     )
 )]
 
-use packed_struct;
-#[macro_use]
-extern crate packed_struct_codegen;
-
 pub mod condcodes;
 pub mod constant_hash;
 pub mod constants;