Implement unionized sections in RGBASM

This touched a lot more code than initially expected, for two reasons.

First, this broke a big RGBASM assumption: that sections are always being
written to at their end. This plus other problems required touching
basically the entirety of `section.c`.

Second, I tried different solutions to solve the above problem, and along
the way I cleaned up many things around. (I believe that keeping this to
"cleanup" commits yields subpar results, and since it's boring they get
postponed anyways.)

RGBLINK support still needs to be added, but this will come next.

test/asm/section-union.err

@@ -0,0 +1,8 @@
+ERROR: section-union.asm(37):
+    Section "test" already declared as union
+ERROR: section-union.asm(37):
+    Section "test" already declared as fixed at $c000
+ERROR: section-union.asm(37):
+    Section "test" already declared as aligned to 256 bits
+ERROR: section-union.asm(37):
+    Cannot create section "test" (3 errors)