Fix some warnings in no_std builds.

The dbg! macro expands to nothing in no_std mode, so variables that are
only used for debugging prompt unused variable warnings.

Also, allow unstable_features in no_std builds, since they use
feature(alloc), which is an unstable feature.

lib/wasm/src/lib.rs

@@ -10,7 +10,8 @@
 //! The main function of this module is [`translate_module`](fn.translate_module.html).
 
 #![deny(missing_docs, trivial_numeric_casts, unused_extern_crates)]
-#![warn(unused_import_braces, unstable_features)]
+#![warn(unused_import_braces)]
+#![cfg_attr(feature = "std", warn(unstable_features))]
 #![cfg_attr(feature = "clippy", plugin(clippy(conf_file = "../../clippy.toml")))]
 #![cfg_attr(feature = "cargo-clippy",
             allow(new_without_default, new_without_default_derive))]