Switch trigram lookup from binary search to a hash table.

Switch trigram lookup from binary search to a hash table.

Binary search was fine when we just wanted simplicity, but for I/O
optimization on rotating media, we want as few seeks as possible.
A hash table with open addressing gives us just that; Robin Hood
hashing makes it possible for us to guarantee maximum probe length,
so we can just read 256 bytes (plus a little slop) for each lookup
and that's it. This kills ~30 ms or so cold-cache.

This breaks the format, so we use the chance to add a magic and
a proper header to provide some more flexibility in case we want
to change the builder.