Redesign file system code

[PatrickKa]

Description

• Separate functions specific to the storage device from the planned littlefs wrapper
• Implement a flash and RAM storage device for littlefs
• Add tests for the RAM storage device
• Add fs::ErrorCode and fs::Result
• Add fs::Mount() to show how I imagine the littlefs wrapper
• Move old file system code in FileSystem.[chi]pp into namespace sts1cobcsw::fs::deprecated

Fixes part of #273
Fixes part of #186
Fixes part of #209 (LfsRam.cpp is enough to run the whole file system code on Linux, but it does not allow changing the functionality at run time)

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 89.28%. Comparing base (ffdc461) to head (aabc47b).

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #278      +/-   ##
==========================================
+ Coverage   88.36%   89.28%   +0.91%     
==========================================
  Files          20       22       +2     
  Lines         576      625      +49     
==========================================
+ Hits          509      558      +49     
  Misses         67       67              

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