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/rept-shift.asm

@@ -0,0 +1,17 @@
+m: macro
+	PRINTT "\1 "
+	REPT 4
+		SHIFT
+	ENDR
+	PRINTT "\1s!\n"
+
+	; Shifting a little more to check that over-shifting doesn't crash
+	SHIFT
+	SHIFT
+	REPT 256
+		SHIFT
+	ENDR
+	PRINTT "\1\n"
+endm
+
+ m This, used, not, to, work

test/asm/rept-shift.err

@@ -0,0 +1,2 @@
+ERROR: rept-shift.asm(17) -> rept-shift.asm::m(14):
+    Macro argument '\1' not defined

test/asm/rept-shift.out

@@ -0,0 +1 @@
+This works!

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

test/asm/unique-id.err

@@ -0,0 +1,19 @@
+warning: unique-id.asm(12) -> unique-id.asm::m(4): [-Wuser]
+    _0
+warning: unique-id.asm(12) -> unique-id.asm::m(5) -> unique-id.asm::m::REPT~1(6): [-Wuser]
+    _1
+warning: unique-id.asm(12) -> unique-id.asm::m(5) -> unique-id.asm::m::REPT~2(6): [-Wuser]
+    _2
+warning: unique-id.asm(12) -> unique-id.asm::m(8): [-Wuser]
+    _0
+warning: unique-id.asm(14) -> unique-id.asm::m(4): [-Wuser]
+    _3
+warning: unique-id.asm(14) -> unique-id.asm::m(5) -> unique-id.asm::m::REPT~1(6): [-Wuser]
+    _4
+warning: unique-id.asm(14) -> unique-id.asm::m(5) -> unique-id.asm::m::REPT~2(6): [-Wuser]
+    _5
+warning: unique-id.asm(14) -> unique-id.asm::m(8): [-Wuser]
+    _3
+ERROR: unique-id.asm(15):
+    Macro argument '\@' not defined
+while expanding symbol "print"