Track max buffer usage in memory channel statistics

[pctablet505]

Adds a max_buffer_used field to MemoryChannelStatistics, tracking the largest number of items the channel's buffer has ever held at once.

The use case: when you need buffering but math.inf isn't a reasonable max_buffer_size, this makes it possible to observe real-world peak usage during a run and pick a sensible limit, instead of guessing.

Closes #1723

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00000%. Comparing base (83b96a3) to head (92093ef).

Additional details and impacted files

@@               Coverage Diff               @@
##                 main        #3474   +/-   ##
===============================================
  Coverage   100.00000%   100.00000%           
===============================================
  Files             128          128           
  Lines           19439        19448    +9     
  Branches         1318         1318           
===============================================
+ Hits            19439        19448    +9     

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

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