Make the adaptors type erased and generic sensitive

As of right now, we must do the following when working with some adaptors:
```cpp
co_await (rstream | map<int>([](int value) {...}) | filter<std::string>([](std::string){...}) ...;
```

We should be only needing to do this:
```cpp
co_await (rstream | map([](int value) {...}) | filter([](std::string) {...}) ...;
```

We shouldn't have to explicitly mention the generic attributes.