feat: implement functions

exl/fn.fzs

@@ -0,0 +1,24 @@
+int fib(int n)
+{
+  if (n <= 1)
+    return n
+
+  return fib(n - 1) + fib(n - 2)
+}
+
+uint sum(uint a, uint b)
+{
+  return a + b
+}
+
+printline("Test #1: fibonacci")
+int result = fib(7)
+if (result == 13)
+  printline("Recurse test passed")
+else printline("Recurse test failed")
+
+printline("Test #2: sum")
+if (sum(2, 2) == 4)
+  printline("Sum test passed")
+else printline("Sum test failed")
+