Improve and rename type_erased_stream -> any_stream_of

[janondrusek]

Following the any_sender_of pattern. There should be no need for a separate call to type_erase in order to get a type_erased_stream return type:

using namespace unifex;
// this should compile
any_stream_of<int> stream() {
  return range_stream{0, 10};
}

Improvements to be made:

1. noexcept correctness
2. reduce / eliminate virtual methods