Replace clones with references

src/bin/problem_3.rs

@@ -8,7 +8,7 @@ fn prime_factors(number: i64) -> Vec<i64> {
     let upper: i64 = (number as f64).sqrt().ceil() as i64;
     for i in 3..upper {
         let mut is_prime = true;
-        for factor in factors.clone() {
+        for factor in &factors {
             if i % factor == 0 {
                 is_prime = false;
                 break;

src/bin/problem_7.rs

@@ -10,7 +10,7 @@ fn nth_prime(nth: i64) -> Option<i64> {
 
     while primes.len() < nth as usize {
         let mut is_prime = true;
-        for prime in primes.clone() {
+        for prime in &primes {
             if i % prime == 0 {
                 is_prime = false;
                 break;