Convert mappings extension to storage unit

[NoComment1105]

Re-done this time. Should be fully functioning.

[gdude2002]

I'm unsure what specifically to do with this one.

The mappings extension is useful for sure, but it's been impossible for me to maintain it for a long time, and @sschr15 seems like he's too busy to do much with it these days.

Additionally, the storage unit system needs a major rework at some point, something which I'm very unlikely to be able to do any time soon.

There is a conflict to resolve here, but I'm left feeling like I didn't give you the right brief to solve this properly - but I don't have the spoons to do a proper review right now anyway, so I'm not sure.

[NoComment1105]

Hm, Ok then. I'll resolve the conflict, then let me know when you have appropriate spoons

[gdude2002]

It's unfortunate that this has gone so long until you managed to get a review in - but when the merge conflicts are solved, things are re-tested, and you're happy with it, we can probably merge @sschr15

[NoComment1105]

@gdude2002 Could this have a final review please? sschr15 has approved it

[sschr15]

Unless the merge commit broke something, I did testing before sending over the patch that was implemented

[gdude2002]

Alright, well then I'll merge it when you've got it to pass the linter.

[gdude2002]

That was an odd one, wasn't it?

Anyway, thanks, I think we finally made it!