message: Make runtime.call_str_ret work on built-in functions
  Prior to this `runtime.call_str_ret("sin", &[Variable::f64(0.0)], &dyon_module);`
  would error with `Error: "Could not find function `sin`"`.
  Now it will return the right result.
diff: @@ -1178,15 +1178,18 @@ impl Runtime {
  use std::cell::Cell;
  let name: Arc<String> = Arc::new(function.into());
  - match module.find_function(&name, 0) {
  - FnIndex::Loaded(f_index) => {
  + let fn_index = module.find_function(&name, 0);
  + if let FnIndex::None = fn_index {
  + return Err(format!("Could not find function `{}`", function));
  + }
  +
  let call = ast::Call {
  alias: None,
  name: name.clone(),
  - f_index: Cell::new(FnIndex::Loaded(f_index)),
  - args: args.iter()
  - .map(|arg| ast::Expression::Variable(Box::new((
  - Range::empty(0), arg.clone()))))
  + f_index: Cell::new(fn_index),
  + args: args
  + .iter()
  + .map(|arg| ast::Expression::Variable(Box::new((Range::empty(0), arg.clone()))))
  .collect(),
  custom_source: None,
  source_range: Range::empty(0),
  @@ -1194,12 +1197,11 @@ impl Runtime {
  match self.call(&call, &module) {
  Ok((Some(val), Flow::Continue)) => Ok(val),
  Err(err) => Err(err),
  - _ => Err(module.error(call.source_range,
  - &format!("{}\nExpected something",
  - self.stack_trace()), self))
  - }
  - }
  - _ => Err(format!("Could not find function `{}`",function))
  + _ => Err(module.error(
  + call.source_range,
  + &format!("{}\nExpected something", self.stack_trace()),
  + self,
  + )),
  }
  }
  