Add support for unions

Unions allow multiple memory allocations (using ds, etc.) to share the
same space in memory.

This allows games to use the same memory for different purposes,
depending on their state.

This also adds documentation on how to use the new UNION, NEXTU, and
ENDU keywords.

src/asm/globlex.c

@@ -331,6 +331,10 @@ struct sLexInitString staticstrings[] = {
 	{"else", T_POP_ELSE},
 	{"elif", T_POP_ELIF},
 	{"endc", T_POP_ENDC},
+	
+	{"union", T_POP_UNION},
+	{"nextu", T_POP_NEXTU},
+	{"endu", T_POP_ENDU},
 
 	{"wram0", T_SECT_WRAM0},
 	{"vram", T_SECT_VRAM},