Support transmission of structs as name:value field pairs

[dylanmckay]

In 19efbfa7fd18613bae6df367b5eb0c8fa4582d52, a new #[protocol(name = "foo")] attribute was introduced for enum variants.

We should support transmission of structures as field_name:field_value pairs so that fields may be added.

When this is done, we should also implement support for the #[protocol(name = "afd")] for each field.