Add support for literals in arithmetic

```asm
XPOS	EQU 1
YPOS	EQU 0
	LDA SHIPS+XPOS,X
	RTS
SHIPS	DFB 00,00
```

Should assemble to `BD 04 80 60 00 00`, but fails with error:

```
expected hex, binary, or decimal literal; got XPOS
```
