Spead2

- Add a fast path for integer items that exactly fit in an immediate.

- Optimise Python code by replacing np.product with a pure Python
implementation.

- Filter out duplicate items from a heap. It is undefined which of a set of
duplicates will be retained (it was already undefined for
:py:class:`spead2.ItemGroup`).

- Changed item versioning on receive to increment version number on each update
rather that setting to heap id. This is more robust to using a single item
or item group with multiple streams, and most closely matches the send path.
- Made the protocol enums from the C++ library available in the Python library
as well.
- Added functions to create stream start items (send) and detect them (recv).

- Added friendlier support for multicast. When a multicast address is passed
to :py:meth:`~spead2.recv.Stream.add_udp_reader`, the socket will
automatically join the multicast group and set :cpp:var:`SO_REUSEADDR` so
that multiple sockets can consume from the same stream. There are also new
constructors and methods to give explicit control over the TTL (send)
and interface (send and receive), including support for IPv6.

- Added in-memory mode to the C++ version of spead2_bench, to measure the
packet handling speed independently of the lossy networking code
- Optimization to duplicate packet checks. This makes a substantial
performance improvement when using small (e.g. 512 byte) packets and large
heaps.

- Fix a data corruption (use-after-free) bug on send side when data is being
sent faster than the socket can handle it.