message: Add a test of eval_function
  Summary: I realised there wasn't a single test for this. Add one.
diff: use crate::{
  self as starlark,
  assert::{self, Assert},
  - environment::{GlobalsBuilder, Module},
  + environment::{Globals, GlobalsBuilder, Module},
  errors::eprint_error,
  eval::Evaluator,
  syntax::{AstModule, Dialect},
  @@ -255,6 +255,25 @@ ys = [lambda x: x + y for y in [4,5,6]]
  );
  }
  +#[test]
  +fn test_eval_function() {
  + let fun = assert::pass(
  + r#"
  +def fun(a, y, x = 1):
  + return str((a, y, x))
  +fun
  +"#,
  + );
  + let env = Module::new();
  + let globals = Globals::default();
  + let mut eval = Evaluator::new(&env, &globals);
  + let hello = env.heap().alloc("hello");
  + let v = eval
  + .eval_function(fun.value(), &[Value::new_int(8)], &[("y", hello)])
  + .unwrap();
  + assert_eq!(v.unpack_str(), Some("(8, \"hello\", 1)"))
  +}
  +
  #[test]
  fn test_nested_def() {
  assert::is_true(
  