Implement GPT support

[bnuuydev]

A few things need doing before this is good to merge:

• Write some tests
• Decide which partition types to define (and add their definitions)
• Some written documentation could be good, but I've given a syntactic definition of how to use it to start with :)
• Really the GPT code should call into the MBR code to make the protective MBR. I can have a look in a few days at making this change, but happy for someone else to if they get to it before I do. I forgot I already did this.
• Partition attributes can't be set (custom attributes pose a problem here) (but this can be forgone honestly; it's pretty niche and the current code is sufficient for bootable disk images, which is the main purpose of this I guess)
• Only supports GPT with a protective MBR for now, no hybrid MBR. Hybrid MBRs are problematic for a number of reasons, so it might be best just to forget about them for now.

The code might not compile since as I was working on master. I think I've made sure the code is right now, but be sure that they are correct before merging :)

[Khitiara]

mostly LGTM (also scooped me, nice), got a few minor nits is all

[Khitiara]

also if you want and run out of time for this I can work on any finishing stuff

[bnuuydev]

Would appreciate that! Sorry for stepping on your toes a bit with this btw, I'd written it a few weeks ago but had been too busy to make the PR >_<

[Khitiara]

no worries, you got a heck of a lot further than i did

[Khitiara]

addressed the nits and pushed an update to the PR

[bnuuydev]

Also make sure not to merge this before we have some tests. I'll write some up in a couple of days if you don't get to it first :)

[Khitiara]

I'll also be switching my os over to this pr branch at some point in a few days to try it out in a real scenario