breaking changes in 0.14

[ubnt-intrepid]

• remove the server and test implementation from the core crate
  • use izanami instead
• remove a lifetime parameter from Endpoint (Remove lifetime parameter from Endpoint #383)
  • we should wait for the implementation of generic associated types (GAT) to be completed.
• Add a type parameter to Endpoint to generalize the type of request bodies:

  trait Endpoint<Bd> {
      fn apply(&self, cx: ApplyContext<'_, Bd>) -> ApplyResult<Self::Future>;
  }

• Introduce a trait that represents a Future with the explicit request information:

  trait Future<Bd> {
      type Ok;
      type Error;
  
      fn poll_ready(&mut self, cx: &mut FutureContext<'_, Bd>) -> Poll<Self::Ok, Self::Error>;
  }