Revamp macro arg system

This should significantly improve performance: on pokecrystal builds, perf
reported as much CPU time spent on `yyparse` as on `sym_UseNewMacroArgs`
Measurements show ~6 seconds of improvement on that codebase.

This also fixes #321, as a bonus, due to saner management!

test/asm/unique-id.asm

@@ -0,0 +1,15 @@
+print EQUS "WARN \"\\@\""
+
+m: macro
+    print
+    REPT 2
+    	print
+    ENDR
+    print
+endm
+	; TODO: Ideally we'd test now as well, but it'd cause a fatal error
+	;print
+	m
+	;print
+	m
+	print