implement StapledMemoryChannel

[belm0]

partially addresses #719

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.64103%. Comparing base (4f1e3e8) to head (3f1c732).
⚠️ Report is 2840 commits behind head on main.

❌ Your project check has failed because the head coverage (99.64103%) is below the target coverage (100.00000%). You can increase the head coverage or adjust the target coverage.

Additional details and impacted files

@@                 Coverage Diff                 @@
##                main       #1784         +/-   ##
===================================================
+ Coverage   99.64021%   99.64103%   +0.00082%     
===================================================
  Files            114         114                 
  Lines          14453       14486         +33     
  Branches        1105        1106          +1     
===================================================
+ Hits           14401       14434         +33     
  Misses            37          37                 
  Partials          15          15                 

Files with missing lines	Coverage Δ
trio/__init__.py	100.00000% <ø> (ø)
trio/_channel.py	100.00000% <100.00000%> (ø)
trio/tests/test_channel.py	100.00000% <100.00000%> (ø)

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[pquentin]

It looks like codecov was down when you submitted that pull request. Closing/reopening

[pquentin]

@belm0 Codecov now complains about aclose() not being tested

[pquentin]

3.9.0-rc2 stalled when installing dependencies, closing/reopening...

[belm0]

@njsmith please take a look

I started with StapledMemoryChannel rather than StapledChannel, since my use case is needing the nowait methods. Not sure whether it's worth making a MemoryChannel abc.