Macro impl_into_lua

macro_rules! impl_into_lua {
    (
        $ty:ty {
            $( $field:ident $(=> $conv:expr)? ),* $(,)?
        }
    ) => { ... };
    (@assign $tbl:ident $field:ident) => { ... };
    (@assign $tbl:ident $field:ident => $conv:expr) => { ... };
}

Implements IntoLua for a struct.

impl_into_lua! {
    FunctionTypeValue {
        location,
        args_struct,
        // closure will apply to whatever the field name itself is
        return_type => return_type.map(|b| *b),
        item_id,
    }
}