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

src/asm/rgbasm.5

@@ -799,6 +799,14 @@ Note that strings are not zero-terminated!
 DB 1,2,3,4,"This is a string"
 .Ed
 .Pp
+.Ic DS
+can also be used to fill a region of memory with some value.
+The following produces 42 times the byte $FF:
+.Pp
+.Bd -literal -offset indent
+DS 42, $FF
+.Ed
+.Pp
 Alternatively, you can use
 .Ic DW
 to store a list of words (16-bit) or