Improve `RequestStream`s ergonomics

[stammw]

As discussed in #56, it is not possible to name the full RequestStream type without frame::FrameStream being externally accessible. Namely, the return value of:

   // h3/src/client.rs
    pub async fn send_request(
        &mut self,
        req: http::Request<()>,
    ) -> Result<RequestStream<FrameStream<C::BidiStream>>, Error> {

Cannot be stored in a struct or passed around in functions params without FrameStream being public.

I see multiple solutions to this:

• The best one in my opinion: extract a trait and return an impl RequestStream.
  • Pros:
    • Makes the type opaque, so future proof in case of API changes.
  • Cons:
    • That's not possible until async traits are a thing.
    • Users will have to Box it if they need to store this in a struct.
• Make frame public or reexport FrameStream in the crate's scope.
  • Pros:
    • It's easy to implement
  • Cons:
    • The type is still quite long to write in downstream code
    • FrameStream is externally available but will never be used directly
• Create type aliases like type RequestStream = RequestStreamImpl<FrameStream<C::BidiStream>>
  • Pros:
    • It's easy to implement
  • Cons:
    • It would introduce another type if we want to split the streams into a reader and a writer: type SendStream = RequestStreamImpl<C::SendStream<B>>

[stammw]

Fixed in #63