As announced in #228, this commit migrates all advisories to the new V2
format, which splits version information into a separate section, and
now has a structure which corresponds to the internal code structure of
the `rustsec` crate.
This is a breaking change for users of `cargo-audit` < 0.9, and anyone
who has written a 3rd party advisory format parser.
Nobody knows what "dwf" is, and the data isn't presently consumed or
surfaced by the `rustsec` crate, so we (hopefully) can rename it without
breaking anything.
