Introduce local labels in hello-world.asm

[Rangi42]

Fixes #153

This seems to me like a reasonable balance of introducing a not-too-complex concept at a not-too-late time.

Earlier on in this "Memory" section, there's a lot of complexity that I'd consider to be more premature and less useful than local labels. (For example, even noting the existence of "SRAM", "OAM", "HRAM", etc in the memory map; or talking about "the chip selector" and how "it’s crucial to keep in mind that different addresses may be backed by different memory chips" -- I'm not saying those are bad things to know, but they certainly aren't applicable this early, whereas local labels would be very applicable to alleviate those long wordy labels I listed in the issue.)

I am not updating the larger projects in parts II and III to use local labels, but with this PR, they'd at least be an option for anyone maintaining that code.

[Rangi42]

Two maintainers (@ISSOtm and myself) agreed to revert this change. I still stand by it being an improvement, but it was unexpectedly merged in the middle of ongoing disagreement/discussion about the idea.

I'm not going to be making further piecemeal changes to gb-asm-tutorial. I've proposed a broader restructure under which my suggestions might be agreeable as a compromise. Any further discussion can go there in #156.

[avivace]

@Rangi42 ISSO is not a maintainer of this repository (see pinned #113 ).

If they want to go back at being one (with the linked responsibilty and expected commitment) I would be very happy.

The scope of this PR is not a 'broader discussion' (which we can gladly have in the appropriate issues/PR). This is not the place for ' How to structure the tutorial's chapter progression '. This is the place to introduce a nice, concise and pragratic explanation for a concept. Stop trying to boil oceans.

We can later move the concept (once #156 goes somewhere) in other parts of the tutorial or iterate on the approach. I'd rather have this here than not have it all or blocked behind n other dependencies.