Allows merging async Streams of different output type.

It's very similar to Tokio's StreamMap, except that it doesn't require the streams to have the same output type. This can be usefull when you don't know what type of streams should be combined, acting as a runtime dynamic select.

Not a zero-cost-abstraction

Since we don't know what types of outputs the streams will generate, the generated output will be a StreamMapAnyVariant, a newtype around Box<dyn Any>. As a result, we rely on dynamic dispatching to transform it back into the desired output. Benching shows that it's 2x as slow as a StreamMap or Tokio's select macro.