vlib/math: Add a pure V backend for vlib/math (#11267)

vlib/math/factorial_test.v

@@ -0,0 +1,13 @@
+module math
+
+fn test_factorial() {
+	assert factorial(12) == 479001600
+	assert factorial(5) == 120
+	assert factorial(0) == 1
+}
+
+fn test_log_factorial() {
+	assert log_factorial(12) == log(479001600)
+	assert log_factorial(5) == log(120)
+	assert log_factorial(0) == log(1)
+}