cl-interpret: Give the interpreter a little love

And stop copying strings around.

sample-code/fib.cl

@@ -1,16 +1,29 @@
 // Calculate Fibonacci numbers
 
 fn main() {
+    for num in 0..=30 {
+        print("fib(", num, ") = ", fib_iterative(num))
+    }
     for num in 0..=30 {
         print("fib(", num, ") = ", fib(num))
     }
 }
 
-/// Implements the classic recursive definition of fib()
+/// The classic recursive definition of fib()
 fn fib(a: i64) -> i64 {
     if a > 1 {
         fib(a - 1) + fib(a - 2)
-    } else {
-        1
-    }
+    } else a
+}
+
+/// The classic iterative algorithm for fib()
+fn fib_iterative(n: i64) -> i64 {
+    let mut a = 0;
+    let mut b = 1;
+    let mut c = 1;
+    for _ in 0..n {
+        a = b;
+        b = c;
+        c = a + b;
+    } else a
 }