Refactor app filesystem code for abstraction / remove redundancy

[anonimal]

By submitting this issue, I confirm the following:

• I have read and understood the contributor guide.
• I have checked that the issue I am reporting can be replicated or that the feature I am suggesting is not present.
• I have checked opened or recently closed pull requests for existing solutions/implementations to my issue/suggestion.

Place an X inside the bracket to confirm

• I confirm.

---

Discussed after #329, referencing #98:

&anonimal EinMByte: Hmm. Ok, so ideally all current filesystem code in app should be in core and we just need a way to interface to it through app?
* anonimal should actually pull up the code
&anonimal lol, I think I answered my own question.
&anonimal So, since app is the 'the third wheel', e.g., the throw-away piece that's only needed to run as stand-alone, EinMByte we could just abstract the calls to *something* in core, and from there within core point to the real filesystem implementation.
&anonimal EinMByte: ^ sound about right?
+EinMByte maybe, but make sure that core know nothing about app
+EinMByte (in think that was the main reason for how it's currently done)
&anonimal Yeah, exactly.
&anonimal But the problem with what we have now is that its redundant code that is confusing to maintain.