cl-interpret: Give the interpreter a little love

And stop copying strings around.

sample-code/fizzbuzz.cl

@@ -1,11 +1,11 @@
 // FizzBuzz, using the unstable variadic-`print` builtin
 
 fn main() {
-    fizz_buzz(10, 20)
+    fizzbuzz(10, 20)
 }
 
 // Outputs FizzBuzz for numbers between `start` and `end`, inclusive
-fn fizz_buzz(start: i128, end: i128) {
+fn fizzbuzz(start: i128, end: i128) {
     for x in start..=end {
         print(if x % 15 == 0 {
             "FizzBuzz"