================================================================================
Simple spawn expression
================================================================================
spawn foo()
--------------------------------------------------------------------------------

(source_file
  (simple_statement
    (spawn_expression
      (call_expression
        (reference_expression
          (identifier))
        (argument_list)))))

================================================================================
Simple spawn expression with function literal
================================================================================
spawn fn () {
    return
}()
--------------------------------------------------------------------------------

(source_file
  (simple_statement
    (spawn_expression
      (call_expression
        (function_literal
          (signature
            (parameter_list))
          (block
            (return_statement)))
        (argument_list)))))

================================================================================
Simple spawn expression with assignment
================================================================================
a := spawn foo()
--------------------------------------------------------------------------------

(source_file
  (simple_statement
    (var_declaration
      (expression_list
        (reference_expression
          (identifier)))
      (expression_list
        (spawn_expression
          (call_expression
            (reference_expression
              (identifier))
            (argument_list)))))))
