math: sqrti, powi, factoriali (#12072)

vlib/math/math_test.v

@@ -946,3 +946,15 @@ fn test_large_tan() {
 		assert soclose(f1, f2, 4e-8)
 	}
 }
+
+fn test_sqrti() {
+	assert sqrti(i64(123456789) * i64(123456789)) == 123456789
+	assert sqrti(144) == 12
+	assert sqrti(0) == 0
+}
+
+fn test_powi() {
+	assert powi(2, 62) == i64(4611686018427387904)
+	assert powi(0, -2) == -1 // div by 0
+	assert powi(2, -1) == 0
+}