Log Message:
Import hs-hashed-storage3-0.3.9 as wip/hs-hashed-storage3.

Support code for reading and manipulating hashed file storage (where
each file and directory is associated with a cryptographic hash, for
corruption-resistant storage and fast comparisons).

The supported storage formats include darcs hashed pristine, a plain
filesystem tree and an indexed plain tree (where the index maintains
hashes of the plain files and directories).