FEAT: new natives ARCTANGENT2 and ATAN2

ATAN2 is direct mapping to libc's ATAN2 function without any conversion so does not accept integer types as an input.

ARCTANGENT2 is more user friendly (but little bit slower) - it uses pair! type as an input of the coordinates and result is in degrees by default (may be changed using /radians refinement)

Example output:
```
>> arctangent2 10x-10
== -45.0

>> arctangent2/radians 10x-10
== -0.7853981633974483

>> atan2 -10.0 10.0
== -0.7853981633974483

```

NOTE: notice that ATAN2 is using the input in classic order (Y and X)

Implements wish: metaeducation/rebol-issues#882

Author: Oldes

src/core/n-math.c

@@ -183,6 +183,23 @@ enum {SINE, COSINE, TANGENT};
 	return R_RET;
 }
 
+/***********************************************************************
+**
+*/	REBNATIVE(arctangent2)
+/*
+//	arctangent2: native [
+//		{Returns the angle of the point, when measured counterclockwise from a circle's X axis (where 0x0 represents the center of the circle). The return value is in interval -180 to 180 degrees.}
+//		point [pair!] "X/Y coordinate in space"
+//		/radians "Result is in radians instead of degrees"
+//	]
+***********************************************************************/
+{
+	REBDEC dval = atan2(VAL_PAIR_Y(D_ARG(1)), VAL_PAIR_X(D_ARG(1)));
+	if(!D_REF(2)) dval *= 180.0 / pi1; //to degrees
+	SET_DECIMAL(D_RET, dval);
+	return R_RET;
+}
+
 
 // Follows faster trigonometric functions (without conversions and bounds checks)
 #ifndef USE_NO_INFINITY //use these functions only with INFINITY support (cos/sin may return 1.#NaN value)!
@@ -271,6 +288,21 @@ enum {SINE, COSINE, TANGENT};
 	return R_RET;
 }
 
+/***********************************************************************
+**
+*/	REBNATIVE(atan2)
+/*
+//	atan2: native [
+//		{Returns the angle of the point y/x in the interval [-pi,+pi] radians.}
+//		y [decimal!] "The proportion of the Y-coordinate"
+//		x [decimal!] "The proportion of the X-coordinate"
+//	]
+***********************************************************************/
+{
+	SET_DECIMAL(D_RET, atan2(VAL_DECIMAL(D_ARG(1)), VAL_DECIMAL(D_ARG(2))));
+	return R_RET;
+}
+
 #endif //!USE_NO_INFINITY