Macro define_input

macro_rules! define_input {
    ( $type_name:ident $( { $($fields:tt)* } )? , 
      $get_function_name:ident, $output_type:ty) => { ... };
    ( $type_name:ident $( { $($fields:tt)* } )? , 
      $get_function_name:ident, cloned $output_type:ty) => { ... };
    ( $type_name:ident $( { $( $field_name:ident : $field_type:ty),* } )? , 
      $get_function_name:ident, $output_type:ty ; $clone_function:expr ; $($output_ref:tt)* ) => { ... };
}

Helper macro to define an input type. This will define the type for you along with an OutputTypeForInput impl and a function wrapper for db.get(ComputationType::new()).