message: Add dummy spans to type errors
diff: @@ -21,7 +21,10 @@ mod normalize {
  assert_eq!(
  normalize(&context, &parse(r"x")),
  - Err(InternalError::UndefinedName(x)),
  + Err(InternalError::UndefinedName {
  + span: Span::start(),
  + name: x,
  + }),
  );
  }
  @@ -213,7 +216,10 @@ mod infer {
  assert_eq!(
  infer(&context, &parse(given_expr)),
  - Err(TypeError::UndefinedName(x)),
  + Err(TypeError::UndefinedName {
  + span: Span::start(),
  + name: x,
  + }),
  );
  }
  @@ -276,7 +282,7 @@ mod infer {
  let given_expr = r"(\a => a) : Type";
  match infer(&context, &parse(given_expr)) {
  - Err(TypeError::ExpectedFunction { .. }) => {},
  + Err(TypeError::UnexpectedFunction { .. }) => {},
  other => panic!("unexpected result: {:#?}", other),
  }
  }
  @@ -303,7 +309,7 @@ mod infer {
  assert_eq!(
  infer(&context, &parse(given_expr)),
  Err(TypeError::NotAFunctionType {
  - expr: Term::Universe(Level::ZERO).into(),
  + span: Span::start(),
  found: Value::Universe(Level::ZERO.succ()).into(),
  }),
  )
  