Remove previously deprecated features (#1777)

- Treating multi-unit strings as numbers
- `rgbasm -Wnumeric-string`
- `ldio [c], a` and `ldio a, [c]` (use `ldh`)
- `ld [c], a` and `ld a, [c]` (use `ldh`)
- `ldh [$xx], a` and `ldh a, [$xx]` (use `$FFxx`)

test/asm/multiple-charmaps.out

@@ -1,11 +1,11 @@
 main charmap
 $0
 newcharmap map1
-$6162
+$0
 newcharmap map2, main
 $0
 setcharmap map1
-$6162
+$0
 newcharmap map3
 $1
 newcharmap map4, map3
@@ -13,7 +13,7 @@ $1
 $2
 setcharmap map3
 $1
-$6364
+$0
 setcharmap main
 $0
 modify main charmap
@@ -24,12 +24,12 @@ pushc
 setcharmap map2
 pushc map3
 $1
-$6364
-$6566
+$0
+$0
 popc
 $0
 popc
-$6162
+$0
 newcharmap map1
 setcharmap map5
 popc