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/macro-arguments.out

@@ -13,12 +13,20 @@
 \2: <2 +  2>
 \3: <3>
 
+'mac a':
+\1: <a>
+
 'mac ':
 
+'mac ,':
+\1: <>
+
+'mac a':
+\1: <a>
+
 'mac a,,':
 \1: <a>
 \2: <>
-\3: <>
 
 'mac ,,z':
 \1: <>
@@ -30,17 +38,21 @@
 \2: <>
 \3: <z>
 
-'mac ,a,b,c,':
+'mac ,a,b,c':
 \1: <>
 \2: <a>
 \3: <b>
 \4: <c>
-\5: <>
 
-'mac ,,x,,':
+'mac ,,x,,,':
 \1: <>
 \2: <>
 \3: <x>
 \4: <>
 \5: <>
 
+'mac E,O,F':
+\1: <E>
+\2: <O>
+\3: <F>
+