Private.FanSourcev ~hash_size ~entry_size n creates a fan_out for an index with hash_size and entry_size, containing n elements.
search t hash is the interval of offsets containing hash, if present.
update t hash off updates t so that hash is registered to be at offset off.
Finalizes the update of the fanout. This is mandatory before any search query.
exported_size t is the size of export t. This does not actually compute the encoding of t.