Macro sequential_id_alloc 

macro_rules! sequential_id_alloc {
    ($ty:ident, $output_ty:ident, $max:expr, $arr_ty:ident) => { ... };
}

A simple sequential id allocator.

The allocator will allocate new ids sequentially from 0 to u8::MAX. Unlike other bitmap / slab allocators, freed ids will not be reused right away, only when the next id pointer wraps around.