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.

include/asm/main.h

@@ -31,6 +31,8 @@ extern uint32_t ulNewMacroSize;
 extern int32_t nGBGfxID;
 extern int32_t nBinaryID;
 
+extern uint32_t curOffset; /* Offset into the current section */
+
 extern struct sOptions DefaultOptions;
 extern struct sOptions CurrentOptions;