It seems possible to get file descriptors through the socket event
callback that are not sockets. Possibly c-ares using an epoll like
interface? As anyio now accepts non-socket file descriptors for
read/write watching this can be handled.

Closes #5

Split the type variable of the protocol into two for different return
types of the `get_update` and `completed` methods.

Rename `konnect.curl.abc.Request` methods:

- `has_response()` -> `has_update()`
- `response()` -> `get_update()`

Closes #1

- Move methods that didn't need to be methods out of Mutli class.
- Use individual send/recv streams instead of a stapled stream, as
  these type check better; send end needs contravariant argument type,
  recv end needs covariant return type, stapled stream could only be
  invariant.

Intermediate responses were being ignored until at least one request
was fully complete.

Mostly for the typing improvement made to `create_memory_object_stream`