Translate ByteString in WebIDL to [u8] (#505)

In arguments take `&[u8]` and in return value return `Vec<u8>`. Should help fill
out a few more APIs on `Header` and `Response`!

crates/backend/src/lib.rs

@@ -1,6 +1,8 @@
 #![recursion_limit = "256"]
 #![cfg_attr(feature = "extra-traits", deny(missing_debug_implementations))]
 
+#[macro_use]
+extern crate log;
 extern crate proc_macro2;
 #[macro_use]
 extern crate quote;