Compress filenames with zstd.

Compress filenames with zstd.

Make blocks of 32 and 32 filenames, and compress then with zstd -6
(the level is fairly arbitrarily chosen). This compresses the repetitive
path information very well, and also allows us to have shorter posting
lists, as they can point into the blocks (allowing dedup).

32 was chosen after eyeballing some compressed sizes, looking for
diminishing returns and then verifying it didn't cost much in terms
of search performance.