This is an unmodified copy of twenty_first::storage at twenty-first
revision 890c451e4e513018d8500bedcd5bf76dd0bafdd9 (master)

It is not yet incorporated into the build.

fixes error:
  In order to be transferred, a Block must have a non-None proof field.

ProofType enum enables specifying/transferring an unimplemented Proof.
This is only temporary.

BlockType enum enables specifying Genesis vs Standard block.

A Standard block has a ProofType
The Genesis block has no ProofType

Under crate::locks we previously had:
 - sync
 - tokio

Each contains an impl of AtomicRw and AtomicMutex, with basically the
same API.  Yet:

 - sync refers to synchronous locks, ie sync vs async.
 - tokio refers to the tokio::sync lock implementation.

So the names are referring to different things.  Instead we change it to:

 - std
 - tokio

Now each refers to a lock implementation, ie std::sync and tokio::sync.

note: we could instead have changed `tokio` to `async`, but then there
might be multiple async lock impls to choose from.  So it seems cleanest
to use the name of each impl.

Moved this benchmark over from twenty_first.

Presently unable to build the twenty_first db_* bench tests because
the storage layer is now async and the divan bench crate doesn't yet
support async.  However, it may soon, see:

nvzqz/divan#39