Introduce a bunch of small new headers

[AngheloAlf]

• New headers:
  • gfxalloc.h
  • graph.h
  • z64elf_message.h
    • I'm not totally convinced by this header. I'm open to better suggestions.
  • z64inventory.h
  • z64olib.h
• Move Gfx_DrawDListOpa and Gfx_DrawDListXlu to gfx.h
• Move Regs_InitData to regs.h

z_olib.c needs to include z64actor.h because of PosRot, which feels kinda dumb, we may want to move that struct somewhere else, since it isn't actor specific.

[hensldm]

z_olib.c needs to include z64actor.h because of PosRot, which feels kinda dumb, we may want to move that struct somewhere else, since it isn't actor specific.

Perhaps just in z64math.h since it just combines two of the structs from there already?

[AngheloAlf]

As a small fyi I ended up removing graph.h since only actually two symbols were being used outside of graph.c I moved those two declarations to gfx.h

[hensldm]

As a small fyi I ended up removing graph.h since only actually two symbols were being used outside of graph.c I moved those two declarations to gfx.h

Looking at this again, would we also want to just move the 3 gfxalloc functions there as well?