Defer KI if trio is doing IO

[A5rocks]

Supersedes #1537.

As with that, fixes #151, first step of #733.

Additionally, fixes #2649 (I need to add a test) and part of #3007 (not intentionally, just because it has to be done in order to still raise a KI. I should add a test.).

[A5rocks]

codecov failure doesn't make any sense and seems to be a recent thing. I'm inclined to believe it's some change in how they normalize file names but I don't think it's worth chasing down for now.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 99.98950%. Comparing base (d65d3f6) to head (ff3da6b).

Additional details and impacted files

@@                 Coverage Diff                  @@
##                 main       #3233         +/-   ##
====================================================
- Coverage   100.00000%   99.98950%   -0.01050%     
====================================================
  Files             124         124                 
  Lines           19047       19055          +8     
  Branches         1287        1287                 
====================================================
+ Hits            19047       19053          +6     
- Misses              0           1          +1     
- Partials            0           1          +1     

Files with missing lines	Coverage Δ
src/trio/_channel.py	100.00000% <100.00000%> (ø)
src/trio/_core/_exceptions.py	100.00000% <100.00000%> (ø)
src/trio/_core/_io_epoll.py	100.00000% <100.00000%> (ø)
src/trio/_core/_io_kqueue.py	100.00000% <100.00000%> (ø)
src/trio/_core/_io_windows.py	100.00000% <100.00000%> (ø)
src/trio/_core/_parking_lot.py	100.00000% <100.00000%> (ø)
src/trio/_core/_run.py	99.80080% <100.00000%> (-0.19921%)	⬇️
src/trio/_core/_tests/test_asyncgen.py	100.00000% <100.00000%> (ø)
src/trio/_core/_tests/test_guest_mode.py	100.00000% <100.00000%> (ø)
src/trio/_core/_tests/test_ki.py	100.00000% <100.00000%> (ø)
... and 9 more

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