[PROTOTYPE] C-level reference tracking

[gmalette]

Similarly to msgpack#387 please don't use this code!

In prior versions, I had implemented reference tracking by having a proper Reference type. Roughly speaking, something like:

Reference = Struct.new(:value)

factory.register_type(
  0x22,
  Reference,
  packer: -> (obj, packer) {
    if (id = packer.id(obj))
      packer.write(id)
    else
      packer.track(obj)
      packer.write(nil)
      packer.write(obj.value)
    end
  },
  unpacker: -> (unpacker) {
    if (id = unpacker.read)
      unpacker.references(id)
    else
      unpacker.track(unpacker.read)
    end
  },
)

The main issue with this approach was the amount of short-lived Reference objects required only for serialization.

To avoid these, I moved the handling of the references directly in how each type is encoded (as seen in the benchmark). This is a Light Abuse™️ of messagepack but saves so much space and can properly deserialize entire graphs of objects.