HBFileIO changes

[adam-fowler]

• Added makeDirectory
• writeFile can be used to write arbitrary buffers, or sequences of buffers
• Don't return size from loadFile as it is already available in returned response body

[codecov]

Codecov Report

Attention: 14 lines in your changes are missing coverage. Please review.

Comparison is base (0b33c97) 84.51% compared to head (0573d21) 84.17%.
Report is 8 commits behind head on 2.x.x.

Files	Patch %	Lines
Sources/Hummingbird/Files/FileIO.swift	22.22%	14 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##            2.x.x     #390      +/-   ##
==========================================
- Coverage   84.51%   84.17%   -0.34%     
==========================================
  Files          95       94       -1     
  Lines        5283     5296      +13     
==========================================
- Hits         4465     4458       -7     
- Misses        818      838      +20     

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