Macro register 

macro_rules! register {
    ( $( $field:ident: $type:ty ),* ) => { ... };
    ( $( $field:ident: $type:ty , )+ ) => { ... };
}

Register a type to be deserialized during init

Types or field names may only be registered once

Only crate-local types may be used

The trailing comma is optional

head_empty::register! {
    name: Type,
    name2: Type2,
}