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

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)!

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

test/asm/ds-byte.out.bin

@@ -0,0 +1 @@
+****<2A><><EFBFBD><EFBFBD><EFBFBD>EEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEE