Add the runes to make quick releases
Improve the buffer size when receiving data from network
Test .idx only if present in lib_test/data/
Fix #38

Need a real hashtbl when unpacking a a pack file, not a LRU.

Also add various LRUs to speed-up cache accesses. Not cloning
opam-repository works and take a reasonnable amount of time (although
I'm sure we can optimize the way paked_values are reconstructed).

Also forbid Lwt_unix.files_of_directory to open many files at once.