Function fixed_items 

pub fn fixed_items<const N: usize>(
    value: &[u8],
) -> Result<impl ExactSizeIterator<Item = &[u8; N]> + Clone, ItemError>

Iterate the fixed-size items of a whole-table value with no item length prefix (channel keys, peer public keys, peer key entries).

Fails up front unless the value is an exact multiple of N, so callers can validate before mutating.