rewrite smarter people's euler solutions

examples/puzzles/euler_21.zr

@@ -0,0 +1,21 @@
+func divisors_sum[n: i64] : i64
+    let k: i64 = n
+    let sum: i64 = 1
+
+    for i in 2..k+1
+        let p: i64 = 1
+        while k % i == 0
+            p = p * i
+            k = k / i
+        sum = sum * (p * i - 1) / (i - 1)
+    return sum - n
+
+func main[] : i64
+    let sum: i64 = 0
+
+    for i in 2..10000
+        let d: i64 = divisors_sum(i)
+        if i < d & i == divisors_sum(d)
+            sum = sum + i + d
+    
+    io.println_i64(sum)