Update to RGBDS 0.7.0

[Rangi42]

These new features are used from RGBDS 0.7.0:

• Use SIZEOF and STARTOF for section types, instead of *RAM_Begin/End constants
• Use ENDSECTION at the end of INCLUDEd files which start their own SECTIONs
  • These can be found via:
    for f in `git grep -Elw '^SECTION'`; do git grep -ho "INCLUDE \"$f\""; done
• OAMDMACodeEnd can be .End since ENDL restores scope

Also rgbdscheck.asm is rewritten to be more robust with older versions of RGBDS. (We can't define constants since DEF is required in 0.7.0 but not yet defined in old enough versions; likewise for backslash line continuations, ELIF, etc.)

Note that the object file revision is still 9, so tools/unnamed.py did not need updating.

[vulcandth]

Lets also update the STYLE.md guide to mention we won't be using :: to separate multiple instructions on one line.

[Rangi42]

STYLE.md was last updated two years ago and may need further changes than that; can we just do it separately?

[aaaaaa123456789]

Lets also update the STYLE.md guide to mention we won't be using :: to separate multiple instructions on one line.

This sounds unnecessary. This is already something extremely unusual, to the point "don't do it" is the default in basically every programming community for every language — if someone wants to do this, there's probably a very good reason for it.