Document integrating `HTTPParser` / `NetworkBackend`.

[lovelydinosaur]

In the parsers documentation, let's get to the point where we can demo how to wire up the NetworkBackend and HTTPParser together to create simple one-shot connection requests...

def request(method: str, url: str) -> tuple(int, bytes):
    b = httpx.NetworkBackend()
    u = httpx.URL(url)
    secure = {"https": True, "http": False}[u.scheme]

    with b.connect(u.host, u.port, secure=secure) as stream:
        p = httpx.HTTPParser(stream, mode='CLIENT')
        p.send_method_line(method, u.target, "HTTP/1.1")
        p.send_headers([("Host", u.netloc), ("Connection", "close")])
        p.send_body("")
        protocol, status, reason = p.recv_status_line()
        headers = p.recv_headers()
        s = httpx.HTTPStream(p)
        body = s.read()

    return status, body

status, body = request("GET", "https://www.example.com")

Some points still todo here...

• Neaten up API. for .connect() on NetworkBackend. (eg. secure by default.)
• HTTPStream to accept parser instance. Document this class in the Parser docs.
• Review bytes/str above.