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/raw-string-symbol-errors.asm

@@ -4,7 +4,7 @@ macro m
 endm
 
 assert (#n) == 42
-assert (#s) == $656c6c6f
+assert (#s) == 0
 assert (#m) == 0
 assert (#u) == 0