AsyncIterator bug fixes

[blogle]

Refactor WatchReceiverStream/Debounce/Switch to use __anext__ and default __aiter__ impl.

This consistency ensures that kioto.streams can be used in async for syntax and be the argument to anext.

[codecov]

Codecov Report

❌ Patch coverage is 92.50000% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 87.55%. Comparing base (4c23f7b) to head (75e9496).
⚠️ Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
kioto/streams/impl.py	92.50%	0 Missing and 3 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master      #23      +/-   ##
==========================================
+ Coverage   87.48%   87.55%   +0.07%     
==========================================
  Files          22       22              
  Lines        1278     1278              
  Branches      160      160              
==========================================
+ Hits         1118     1119       +1     
+ Misses        108      107       -1     
  Partials       52       52              

Files with missing lines	Coverage Δ
kioto/channels/impl.py	86.47% <ø> (+0.16%)	⬆️
kioto/streams/impl.py	96.30% <92.50%> (+0.02%)	⬆️

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