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/asmy.y

@@ -884,6 +884,10 @@ ds		: T_POP_DS uconst
 		{
 			out_Skip($2);
 		}
+		| T_POP_DS uconst comma reloc_8bit
+		{
+			out_RelBytes(&$4, $2);
+		}
 ;
 
 db		: T_POP_DB constlist_8bit_entry comma constlist_8bit {

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

src/asm/section.c

@@ -295,11 +295,7 @@ void out_String(char const *s)
 		absByteBypassCheck(*s++);
 }
 
-/*
- * Output a relocatable byte. Checking will be done to see if it
- * is an absolute value in disguise.
- */
-void out_RelByte(struct Expression *expr)
+static void outputExpression(struct Expression const *expr)
 {
 	if (!rpn_isKnown(expr)) {
 		out_CreatePatch(PATCHTYPE_BYTE, expr);
@@ -307,6 +303,26 @@ void out_RelByte(struct Expression *expr)
 	} else {
 		out_AbsByte(expr->nVal);
 	}
+}
+
+/*
+ * Output a relocatable byte. Checking will be done to see if it
+ * is an absolute value in disguise.
+ */
+void out_RelByte(struct Expression *expr)
+{
+	outputExpression(expr);
+	rpn_Free(expr);
+}
+
+/*
+ * Output several copies of a relocatable byte. Checking will be done to see if
+ * it is an absolute value in disguise.
+ */
+void out_RelBytes(struct Expression *expr, int32_t n)
+{
+	while (n--)
+		outputExpression(expr);
 	rpn_Free(expr);
 }