SepReader: Optimize MoveNextAsync

[nietras]

Update row benchmarks.

[codecov]

Codecov Report

Attention: Patch coverage is 91.66667% with 1 line in your changes missing coverage. Please review.

Project coverage is 99.63%. Comparing base (2a71b43) to head (6e16ac6).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
src/Sep/SepReader.IO.Async.cs	90.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #245      +/-   ##
==========================================
- Coverage   99.66%   99.63%   -0.03%     
==========================================
  Files          54       54              
  Lines        4119     4120       +1     
  Branches      464      465       +1     
==========================================
  Hits         4105     4105              
- Misses         11       12       +1     
  Partials        3        3              

Flag	Coverage Δ
Debug	99.41% <83.33%> (-0.05%)	⬇️
Release	99.82% <100.00%> (ø)
macos-latest	93.10% <91.66%> (-0.03%)	⬇️
ubuntu-latest	99.58% <91.66%> (-0.03%)	⬇️
windows-latest	99.58% <91.66%> (-0.03%)	⬇️

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