Implement zero copy writes for TCP socket (sync and async) transports

[francis-clairicia]

What's changed

• Default implementations of send_all_from_iterable() do not concatenate received iterable in one byte buffer.
• Implementations of send_all_from_iterable() for TCP sockets (without SSL) use sendmsg(2) on platforms supporting it (UNIX systems currently).

Unchanged behaviors

• asyncio.Transport.writelines() concatenates the iterable of data by default. AsyncioTransportStreamSocketAdapter will continue to use it.
  • Since CPython 3.12, UNIX implementations of asyncio.Transport use zero-copy writes (see Implement zero copy writes in SelectorSocketTransport in asyncio python/cpython#91166)

[codecov]

Codecov Report

Merging #167 (1d58bb5) into main (8f50e10) will not change coverage.
The diff coverage is 100.00%.

Additional details and impacted files

Flag	Coverage Δ
OS-Linux	85.88% <68.62%> (-0.41%)	⬇️
OS-Windows	85.25% <48.03%> (-0.66%)	⬇️
OS-macOS	85.91% <68.62%> (-0.41%)	⬇️
Py-3.11	99.89% <100.00%> (+<0.01%)	⬆️
Py-3.12	99.89% <100.00%> (+<0.01%)	⬆️
test-functional	86.16% <75.49%> (-0.18%)	⬇️
test-unit	83.62% <100.00%> (+0.23%)	⬆️

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