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.err

@@ -0,0 +1,5 @@
+ERROR: ds-bad.asm(3):
+    Expected constant expression: 'unknown' is not constant at assembly time
+warning: ds-bad.asm(4): [-Wtruncation]
+    Expression must be 8-bit
+error: Assembly aborted (1 errors)!