The build is failing because `keccak` has a very old MSRV of 1.41, which
after merging #40 (which bumped the `ascon` edition to 2021) broke the
build because it can't handle the `edition` key.
As a workaround, this excludes `keccak` from the toplevel workspace so
it doesn't try to read the `ascon` edition key.
Separately, we should probably bump the `keccak` edition to 2021 as well
and release `keccak` v0.2.0, but that is left for future work.
