`keccak` was previously excluded from the toplevel workspace due to its
MSRV of 1.41 and incompatibilities with packages used by `ascon`.
This bumps the edition, changes the version to `0.2.0-pre` to denote it
contains breaking changes (though this is not intended to be a
`0.2.0-pre` release), and bumps MSRV.
With the MSRV bumped, `keccak` can and has been re-added to the toplevel
workspace.
