Add a basic `read_with_timeout` functionallity

[duck-5]

The StreamReader read methods don't have a timeout mechanism, and for various reasons they don't won't it as an part of the core library. Here, though, it seems fit to write a wrapper for the basic read method.
Something like:

async def read_with_timeout(reader, n, timeout):
    start_t = time.time()
    data = b''
    while time.time() - start_t < timeout and len(data) < n:
        try:
            data += await asyncio.wait_for(reader.read(1), timeout=timeout):
        except TimeoutError:
            pass
    return data

Would be very helpful.