cmd/margin: add a new --predict option.

cmd/margin: add a new --predict option.

When --predict is given, it tries to guess the offset in the indexfile of
each hash, based on assumption that the hashes are distributed evenly
throughout the file.  Then it prints the maximum amount by which this guess
deviates from reality.

I was hoping the results would show that the maximum deviation in a typical
midx was less than a page's worth of hashes; that would mean the toplevel
lookup table could be redundant, which means fewer pages hit in the
common case.  No such luck, unfortunately; with 1.6 million objects, my
maximum deviation was 913 hashes (about 18 kbytes, or 5 pages).

By comparison, midx files should hit about 2 pages in the common case (1
lookup table + 1 data page).  Or 3 pages if we're unlucky and the search
spans two data pages.

Signed-off-by: Avery Pennarun <apenwarr@gmail.com>