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-bad.asm

@@ -0,0 +1,4 @@
+SECTION "test", ROM0[0]
+
+	ds unknown, 0
+	ds 1, $100