Add ds cnt, byte syntax

As suggested by https://github.com/rednex/rgbds/issues/350#issuecomment-498030458
The order `count` then `byte` was decided after some discussion:
- First argument consistent with single-arg syntax
- Intuitive at least to some people other than myself
- Consistent with other assemblers, at least ca65

test/asm/ds-byte.asm

@@ -0,0 +1,8 @@
+SECTION "test", ROM0[0]
+
+Label:
+	ds 4, 42
+.other
+	ds 5, .other - Label - 5 ; Expressions should work...
+	ds 60, .last - Label     ; ...even if not constant
+.last