Support Serializable WebIDL attribute with build-time code generation

[jdm]

https://html.spec.whatwg.org/multipage/structured-data.html#serializable-objects

There's a bunch of hand-written code in https://github.com/servo/servo/blob/master/components/script/dom/bindings/structuredclone.rs to support serializing Blob values via postMessage that would need to be duplicated to support other types. It would be better to do the following:

• create a Serializable trait which encompasses the serialization and deserialization steps for a particular interface
• implement the new trait for the Blob type and rewrite the implementation of structuredclone.rs to use this trait
• add a method to the DOMObject trait to return an optional Serializable trait object and rewrite the implementation of structuredclone.rs to rely on this method instead of hardcoding Blob support
• add codegen support for the Serializable WebIDL extended attribute which implements the Serializable trait for the interface by invoking serialization_steps and deserialization_steps methods for the underlying object
• generate the StructuredCloneTags enum and StructuredCloneHolder struct based on all known serializable interfaces, as well as the read_callback and write_callback functions