README: fix typo
README: use simpler language
README: document future work
DESIGN: update to describe the actually implemented design
shared.h: include <stdio.h> for standaloneness
bloom: use do_hash rather than local hash() copy in unit tests
bloom: fixup switch to do_hash
undup: switch to local sha512 rather than linking OpenSSL for sha256

This is an incompatible format change, sha256 hashed filesystems are not
supported after this change.