Improve and rename `type_erased_stream` -> `any_stream_of`

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:

```C++
using namespace unifex;
// this should compile
any_stream_of stream() {
return range_stream{0, 10};
}
```

Improvements to be made:

1. `noexcept` correctness
2. reduce / eliminate `virtual` methods