Add recursive_generator<T> coroutine type

[lewissbaker]

Add a new recursive_generator<T> type that allows for efficient enumeration over a sequence that is defined recursively.

This type would allow you to pass either a T or a recursive_generator<T> to the co_yield expression.
The increment operator on the iterator would then directly resume the leaf-most coroutine rather than the having to resume each coroutine in the stack until the leaf is resumed and then later suspend every coroutine on the stack for each item.

eg.

generator<directory_entry> list_directory(std::string path);

recursive_generator<directory_entry> recursive_list_directory(std::string path)
{
  for (auto& entry : list_directory(path))
  {
    co_yield entry;
    if (entry.is_directory())
    {
      co_yield recursive_list_directory(entry.path());
    }
  }
}

void usage()
{
  for (auto& entry : recursive_list_directory("foo/bar"))
  {
    std::cout << entry.path() << "\n";
  }
}

[lewissbaker]

The iteration should be cancellable at any point by simply destructing the owning recursive_generator<T> object. This should in turn destruct all nested coroutine frames recursively as the automatic variables within each coroutine fame, which should include the recursive_generator for the nested sequence, are destructed as the coroutine is destroyed.

[lewissbaker]

Commit 6247291 added an initial version of recursive_generator.