Initial HTTP/3 implementation

This includes Websocket over HTTP/3.

Since quicer, which provides the QUIC implementation,
is a NIF, Cowboy cannot depend directly on it. In order
to enable QUIC and HTTP/3, users have to set the
COWBOY_QUICER environment variable:

  export COWBOY_QUICER=1

In order to run the test suites, the same must be done
for Gun:

  export GUN_QUICER=1

HTTP/3 support is currently not available on Windows
due to compilation issues of quicer which have yet to
be looked at or resolved.

HTTP/3 support is also unavailable on the upcoming
OTP-27 due to compilation errors in quicer dependencies.
Once resolved HTTP/3 should work on OTP-27.

Because of how QUIC currently works, it's possible
that streams that get reset after sending a response
do not receive that response. The test suite was
modified to accomodate for that. A future extension
to QUIC will allow us to gracefully reset streams.

This also updates Erlang.mk.

src/cowboy_http.erl

@@ -12,6 +12,8 @@
 %% ACTION OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF
 %% OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
 
+%% @todo Worth renaming to cowboy_http1.
+%% @todo Change use of cow_http to cow_http1 where appropriate.
 -module(cowboy_http).
 
 -export([init/6]).
@@ -1531,7 +1533,7 @@ maybe_socket_error(_, Result = {ok, _}, _) ->
 maybe_socket_error(State, {error, Reason}, Human) ->
 	terminate(State, {socket_error, Reason, Human}).
 
--spec terminate(_, _) -> no_return().
+-spec terminate(#state{} | undefined, _) -> no_return().
 terminate(undefined, Reason) ->
 	exit({shutdown, Reason});
 terminate(State=#state{streams=Streams, children=Children}, Reason) ->