Fix IFS compile warnings/errors

[mikee47]

This PR improves the IFS ObjectBuffer implementation to fix 'hidden virtual method' warnings produced by more recent compilers. The class is just a helper/wrapper with a set of custom 'write' methods, so shouldn't have used inheritance in the first place.

Also fixes an 'uninitialised variable' warning in the Spiffs IFS library, identified by running IFS integration tests through valgrind.

[what-the-diff]

PR Summary

• Submodule Update
  The submodule named Components/IFS was updated to a new commit state (36637f4e20), which means that the submodule code has been synchronized with the latest changes.

• Code Refactoring in FileSystem.h
  The FileSystem.h file has undergone a small code modification. Specifically, the way spiffs fs is being declared has changed. Now it is being initialized with empty brackets spiffs fs{}, indicating a more cautious and competent way to deal with object creation and initialization.