reduce redundant code, add tests for reduce and gcd

vlib/math/fraction.v

@@ -87,7 +87,7 @@ pub fn (f1 Fraction) gcd() i64 {
 
 // Fraction method which reduces the fraction
 pub fn (f1 Fraction) reduce() Fraction {
-	cf := gcd(f1.n, f1.d)
+	cf := f1.gcd()
 	return Fraction{f1.n / cf, f1.d / cf}
 }
 

vlib/math/fraction_test.v

@@ -132,4 +132,10 @@ fn test_fraction_equals() {
 	f1 = math.fraction(1,2)
 	f2 = math.fraction(3,4)
 	assert !f1.equals(f2)
+}
+
+fn test_gcd_and_reduce(){
+	mut f := math.fraction(3, 9)
+	assert f.gcd() == 3
+	assert f.reduce().equals(math.fraction(1, 3)) 
 }