Allow trailing commas in bare lists

This applies to macro arguments, DB, DW, DL, DS,
PRINT, PRINTLN, EXPORT, PURGE, and OPT.

It also removes support for empty entries in DB/DW/DL.
(Deprecating it would require keeping parser support,
which is ambiguous with trailing commas.)

Fixes #753

test/asm/trailing-commas.asm

@@ -0,0 +1,25 @@
+SECTION "test", ROM0
+
+mac: MACRO
+	println "\#"
+ENDM
+
+	mac 1,2, 3 , ,5,
+
+	db 1,2,3,
+	dw 4,5,6,
+	dl 7,8,9,
+	ds 10, $a, $b, $c,
+
+	print "Hello", " ",
+	println "world", "!",
+
+spam:
+eggs:
+lobsterThermidor:
+
+	export spam, eggs,
+	purge lobsterThermidor,
+
+	opt boO, g.xX#,
+	dw %ooOOOOoo, `XX##..xx,