FEAT: new native: NAN?

Returns TRUE if the number is Not-a-Number.
Example:
```
>> nan? 1
== false

>> nan? 1.#NaN
== true

>> nan? 1.0 / 0
== false

>> nan? sin 2.0
== false

>> nan? asin 2.0
== true

```

Author: Oldes

src/core/n-math.c

@@ -303,6 +303,20 @@ enum {SINE, COSINE, TANGENT};
 	return R_RET;
 }
 
+/***********************************************************************
+**
+*/	REBNATIVE(nanq)
+/*
+//	nan?: native [
+//		{Returns TRUE if the number is Not-a-Number.}
+//		value [number!]
+//	]
+***********************************************************************/
+{
+	SET_LOGIC(D_RET, isnan(AS_DECIMAL(D_ARG(1))));
+	return R_RET;
+}
+
 #endif //!USE_NO_INFINITY