The client Request now uses the same system as a server Response to track
the write status of headers, and the API has been updated accordingly.
Additionally, the Request constructors have been moved onto the Request object
instead of being top-level hyper functions, as this better namespaces the
client and Server.
Trying to default the type parameters leads to an ICE and strange type errors.
I think this is just due to the experimental state of default type params and
this change can be rolled back when they are fixed.
The client benchmarks did not have to be changed at all for this whole
refactor, and the server benchmark only had to specify a single type parameter,
and only because it writes out the type of Listener, which is not normal
usage.
