Specialize flush to write file to disk

[jishnub]

This makes it convenient to write the file to disk without having to close it.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 91.31%. Comparing base (7b3956e) to head (b2d1589).
Report is 6 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #219      +/-   ##
==========================================
+ Coverage   91.27%   91.31%   +0.03%     
==========================================
  Files           5        5              
  Lines         688      691       +3     
==========================================
+ Hits          628      631       +3     
  Misses         60       60              

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