Allow constructing Quat from two Vector3s

GDScript in Godot 3 doesn't allow having multiple constructors with the
same argument count but different types.

core/variant_call.cpp

@@ -464,6 +464,10 @@ struct _VariantCall {
 	VCALL_LOCALMEM1R(Vector3, limit_length);
 	VCALL_LOCALMEM0R(Vector3, sign);
 
+	static void _call_Vector3_quat_to(Variant &r_ret, Variant &p_self, const Variant **p_args) {
+		r_ret = Quat(p_self.operator Vector3(), p_args[0]->operator Vector3());
+	}
+
 	VCALL_LOCALMEM0R(Plane, normalized);
 	VCALL_LOCALMEM0R(Plane, center);
 	VCALL_LOCALMEM0R(Plane, get_any_point);
@@ -1874,6 +1878,7 @@ void register_variant_methods() {
 	ADDFUNC1R(VECTOR3, VECTOR3, Vector3, reflect, VECTOR3, "n", varray());
 	ADDFUNC1R(VECTOR3, VECTOR3, Vector3, limit_length, REAL, "length", varray(1.0));
 	ADDFUNC0R(VECTOR3, VECTOR3, Vector3, sign, varray());
+	ADDFUNC1R(VECTOR3, QUAT, Vector3, quat_to, VECTOR3, "to", varray());
 
 	ADDFUNC0R(PLANE, PLANE, Plane, normalized, varray());
 	ADDFUNC0R(PLANE, VECTOR3, Plane, center, varray());

doc/classes/Vector3.xml

@@ -207,6 +207,13 @@
 				Returns this vector projected onto the vector [code]b[/code].
 			</description>
 		</method>
+		<method name="quat_to">
+			<return type="Quat" />
+			<argument index="0" name="to" type="Vector3" />
+			<description>
+				Returns the quaternion representing the shortest arc between this vector and [code]to[/code]. These can be imagined as two points intersecting a sphere's surface, with a radius of [code]1.0[/code].
+			</description>
+		</method>
 		<method name="reflect">
 			<return type="Vector3" />
 			<argument index="0" name="n" type="Vector3" />