crypto.rand: add PRNG function read_u64

vlib/crypto/rand/rand_test.v

@@ -4,7 +4,7 @@
 
 import crypto.rand
 
-fn test_crypto_rand() {
+fn test_crypto_rand_read() {
 	no_bytes := 100
 	max_percentage_diff := 20
 
@@ -28,3 +28,16 @@ fn test_crypto_rand() {
 
 	assert diff_percentage <= max_percentage_diff
 }
+
+fn test_crypto_rand_read_u64() {
+	max := u64(200)
+	r1 := rand.read_u64(max) or {
+		assert false
+		return
+	}
+	r2 := rand.read_u64(max) or {
+		assert false
+		return
+	}
+	assert r1 > u64(0) && r2 > u64(0) && r1 < max && r2 < max
+}