Spead2

- Fix segfault in shutdown for :file:`spead2_recv.py` (fixes 56).
- Fix for :py:exc:`TypeError` in Python 3.6 when reading fields that aren't
aligned to byte boundaries.
- Include binary wheels in releases.

- Fix multi-endpoint form of
:py:meth:`spead2.recv.Stream.add_udp_ibv_reader`.

- Rewrite the Python wrapping using pybind11. This should not cause any
compatibility problems, unless you're using the :file:`spead2/py_*.h`
headers.
- Allow passing :cpp:class:`std::shared_ptr<thread_pool>` to constructors that
take a thread pool, with the constructed object holding a reference.
- Prevent constructing a :py:class:`spead2.recv.Stream` with
``max_heaps=0`` (fixes 54).

- Fix rate limiting causing longer sleeps than necessary (fixes 53).

- Disable LTO by default and require the user to opt in, because even if the
compiler supports it, linking can still fail (fixes 51).

- Support multiple endpoints for one :cpp:class:`~spead2::recv::udp_ibv_reader`
(fixes 48).

- Fix compilation on OS X 10.9 (fixes 49)

- Fix :cpp:func:`spead2::ringbuffer<T>::emplace` and :cpp:func:`spead2::ringbuffer<T>::try_emplace`

- Improved error messages when passing invalid arguments to mcdump