FEAT: added support for zero? on bitsets.

`zero?` return TRUE if no bits are set in a bitset!
```
>> zero? make bitset! #{0000}
== true

>> zero? make bitset! #{1000}
== false
```

related to: Oldes/Rebol-issues#1353

Author: Oldes

src/core/n-math.c

@@ -803,5 +803,9 @@ enum {SINE, COSINE, TANGENT};
 		VAL_SET(val, type);
 		if (Compare_Values(D_ARG(1), D_ARG(2), 1)) return R_TRUE;
 	}
+	else if (type == REB_BITSET && Is_Zero_Bitset(VAL_SERIES(D_ARG(1)))) {
+		return R_TRUE;
+	}
+		
 	return R_FALSE;
 }

src/core/t-bitset.c

@@ -493,6 +493,22 @@
 	return PE_BAD_SET;
 }
 
+/***********************************************************************
+**
+*/	REBOOL Is_Zero_Bitset(REBSER *bset)
+/*
+**		Check if all bits are unset.
+**
+***********************************************************************/
+{
+	REBCNT i;
+	REBYTE *bp = BIN_HEAD(bset);
+
+	for(i = 0;  i < SERIES_TAIL(bset); i++) {
+		if(bp[i] != 0) return FALSE;
+	}
+	return TRUE;
+}
 
 /***********************************************************************
 **