Breaking: refactor structured clone into serialize/deserialize

This rewrites most of the cloneable and transferable object
infrastructure to better reflect the reality that structured cloning
requires separate serialization and deserialization steps, instead of a
single operation that creates a new object in the target Realm. This is
most evident in the case of MessagePorts, as noted in #2277. It also
allows us to avoid awkward double-cloning with an intermediate
"user-agent defined Realm", as seen in e.g. history.state or IndexedB;
instead we can simply store the serialized form and later deserialize.

Concretely, this:

* Replaces the concept of cloneable objects with serializable objects.
  For platform objects, instead of defining a [[Clone]]() internal
  method, serializable platform objects are annotated with the new
  [Serializable] IDL attribute, and include serialization and
  deserialization steps in their definition.
* Updates the concept of transferable objects. For platform objects,
  instead of defining a [[Transfer]]() internal method, transferable
  platform objects are annotated with the new [Transferable] IDL
  attribute, and include transfer and transfer-receiving steps.
  Additionally, the [[Detached]] internal slot for such objects is now
  managed more automatically.
* Removes the StructuredClone() abstract operation in favor of separate
  StructuredSerialize() and StructuredDeserialize() abstract operations.
  In practice we found that performing a structured clone alone is never
  necessary in specs. It is always either coupled with a transfer list,
  for which StructuredCloneWithTransfer() can be used, or it is best
  expressed as separate serialization and deserialization steps.
* Removes IsTransferable() and Transfer() abstract operations. When
  defined more properly, these became less useful by themselves, so they
  were inlined into the rest of the machinery.
* Introduces StructuredSerialzieWithTransfer() and
  StructuredDeserializeWithTransfer(), which can be used by other
  specifications which need to define their own postMessage()-style
  algorithm but for which StructuredCloneWithTransfer() is not
  sufficient.

Closes #785. Closes #935. Closes #2277. Closes #1162. Sets the stage for
#936 and #2260/#2361.