Adds new types: bufring::{BufRing, Builder}, bufgroup:{BufX, Bgid, Bid}

    (BufX was chosen as the name of the buffer type of this pool
    simply to make it short and easy to grep for.)

    bufgroup may be better named provbuf, or provided_buffers, down the
    road, especially if we intent to support the other, older, much less
    efficient provided buffer mechanism. But the liburing team
    specifically recommends the buf_ring implementation for ease of use
    and performance reasons so we may not want to incur the overhead of
    supporting both forms.

    bufring could become a trait down the road, where different BufRing
    implementations can be provided. But to avoid the extra decision
    making that comes with designing traits, and to keep the code
    relatively easy to follow, these are all concrate types.

Adds register and unregister functionality to BufRing and to the
tokio_uring driver.

Adds experimental recv, recv_provbuf methods to TcpStream.

    recv_provbuf is a purposefully ugly name. It will be replaced by
    something ... maybe a recv builder sometimme soon.

    Whatever is chosen would then be copied for Udp, Unix and for all
    the other operations that optionally take a provided buffer pool id.

Adds 'net' unit tests. All are admittedly simple ping/pong tests where
only the clients' received lengths are checked, not the actual data.

Adds a tests/common area.

Adds a test case that uses two std::threads, where each thread runs its
own tokio_uring runtime and its own buf_ring provided buffer pool.

    The two-thread case is made long, with many clients, sending many
    large messages to the server and getting them back, in order to see
    gross performance impacts when changing things. It takes 3s on my
    machine. Before going into mainline, the numbers would be changed so
    it took no more than the other unit tests, so about 10ms.

Many TODOs left to cleanup. Primarily Safety rationalizations.

The buffer allocation is made as a single allocation, along with the
ring.

The buffer group id, bgid, also sometimes called the provided buffer
group id, is manually selected through the builder api. There is no
mechanism to pick one automatically. That could be added later but is
not really necessary for this feature to be useful.

This first implementation is without traits and without public
interfaces that would let a user create a different kind of buf_ring or
a different kind of `provided buffers` pool.

There's a question to the liburing team outstanding about how to
interpret an unexpected cqe result of res=0 and flags=4.

The first operation that supports streaming CQE results.

  Adds a Streamable trait, along the lines of the Completable trait.
    Comes with stream_next and stream_complete methods.

  Adds MultiCQEStream struct, along the lines of the MultiCQEFuture
  struct.

  Adds a submit_op_stream along the lines of submit_op.

  Adds poll_next_op along the lines of poll_op.

  The Lifecycle gets two additional methods: take_index, and data.

    take_index: returns the Lifecycle index and replaces it with
    usize::MAX to show the Lifecycle is no longer represented in the
    slab. This feature only used by the new poll_next_op.

    data: returns a reference to the Lifecycle's data, to be able to use
    it for the Streamable's stream_next calls which only require a
    reference. Ownership is still transferred in stream_complete.

Adds the io/recv_multi operation.

The tcp stream recv_multi is a function that
bridges the private types with a public function.

There is no cancel yet for the multishot command but the code can be
written to break out of the loop. Also, when the connection is closed,
the command should fail. It's not tested, but unregistering the buf_ring
might cancel the command too - but maybe not.

Unit tests:

  net unit tests and helper functions return io::Return

  add recv_multi cases

  add BufRingProps to net unit tests - to quiet empty buffer warnings
  which are intentional in some tests

And adds a unit test to tests/net.