For BigFiles we'll needs to maintain `{} offset-in-file -> void *` mapping. A
hash or a binary tree could be used there, but since we know files are
most of the time accessed sequentially and locally in pages-batches, we
can also organize the mapping in batches of keys.

Specifically offset bits are so divided into parts, that every part
addresses 1 entry in a table of hardware-page in size. To get to the actual
value, the system lookups first table by first part of offset, then from
first table and next part from address - second table, etc.

To clients this looks like a dictionary with get/set/del & clear methods,
but lookups are O(1) time always, and in contrast to hashes values are
stored with locality (= adjacent lookups almost always access the same tables).