Add SHA-256 and SHA-512 hashes

Most security-conscious users expect to find SHA-256 hashes of files,
since both MD5 and SHA-1 are considered weak.  SHA-512 additionally has
the benefit that it is significantly faster than SHA-256 on 64-bit
systems, processing twice as much data per chunk with only 125% of the
operations.  Generate both SHA-256 and SHA-512 hashes anywhere we
provide SHA-1 hashes.