Use PipeChannels or FdStreams to make worker processes natively async

[richardsheridan]

This PR uses #1782 to make the workers in #1781 more natively async. This is a separate PR because:

• Worker processes #1781 is actually feature complete as far as I can tell even with the threads, which don't leak during cancellation.
• It adds substantial platform-dependent complexity into the new WorkerProc classes which would previously have been hidden within multiprocessing.Pipe.
• It needs to reach into the internals of the handle/fd holder objects to avoid the problems of TrioInternalError while using a PipeReceiveStream #1767.

However, it provides two attractive opportunities:

• The pickler is exposed and available to be exchanged for something less picky to transfer objects between processes.
• Avoiding the use of worker threads as much as possible (This speeds up certain microbenchmarks, and ideally it could also use a thread-free replacement for trio.lowlevel.WaitForSingleObject)

These commits should be straightforward to rebase on top of #1781 and #1782, if those are accepted. Until then, this PR can just serve as a manifesto for the other PRs.

[codecov]

Codecov Report

Merging #1783 (4ba5c30) into master (f751897) will decrease coverage by 0.07%.
The diff coverage is 97.30%.

@@            Coverage Diff             @@
##           master    #1783      +/-   ##
==========================================
- Coverage   99.64%   99.57%   -0.08%     
==========================================
  Files         114      117       +3     
  Lines       14503    14932     +429     
  Branches     1105     1130      +25     
==========================================
+ Hits        14451    14868     +417     
- Misses         37       45       +8     
- Partials       15       19       +4     

Impacted Files	Coverage Δ
trio/_worker_processes.py	92.10% <92.10%> (ø)
trio/__init__.py	100.00% <100.00%> (ø)
trio/_core/_windows_cffi.py	100.00% <100.00%> (ø)
trio/_windows_pipes.py	100.00% <100.00%> (ø)
trio/tests/test_windows_pipes.py	100.00% <100.00%> (ø)
trio/tests/test_worker_process.py	100.00% <100.00%> (ø)
trio/to_process.py	100.00% <100.00%> (ø)

[richardsheridan]

Closing in favor of trio-parallel. Feel free to reopen someday though!