Code review feedback.

* Move `Module::compile` to `Engine::precompile_module`.
* Remove `Module::deserialize` method.
* Make `Module::serialize` the same format as `Engine::precompile_module`.
* Make `Engine::precompile_module` return a `Vec<u8>`.
* Move the remaining serialization-related code to `serialization.rs`.

tests/all/module_linking.rs

@@ -39,7 +39,7 @@ fn compile() -> Result<()> {
     assert_eq!(m.imports().len(), 0);
     assert_eq!(m.exports().len(), 0);
     let bytes = m.serialize()?;
-    Module::deserialize(&engine, &bytes)?;
+    Module::new(&engine, &bytes)?;
     assert_eq!(m.imports().len(), 0);
     assert_eq!(m.exports().len(), 0);
     Ok(())