Add tube multiplexing to pwntools

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What this task asks for

Add a TubeMultiplexer and MuxChannel API for multiple bidirectional logical channels over one tube, with flow control and close propagation. `TubeMultiplexer(underlying, max_channels=256, high_water_mark=1048576, low_water_mark=262144)` must reject non-tube arguments with `TypeError`, reject `max_channels` outside `[1, 65535]` with `ValueError`, and reject `low_water_mark > high_water_mark` with `ValueError`. Channel IDs must be integers in the range `[1, 65535]`. If a thread is blocked in `accept_channel` when `close()` is called, it must be unblocked with `EOFError`. `MuxChannel` must support half-close via `shutdown('send')`: after half-closing the send direction, further sends must raise `EOFError` while receives continue to work. The `Buffer` class must gain `set_watermarks(high=None, low=None)` (raising `ValueError` if `low > high`), plus properties `high_water`, `low_water`, `over_high_water` (True when size >= high, False if unset), and `under_low_water` (True when size <= low, False if unset). Calling `mux(**kwargs)` on any tube instance must return a `TubeMultiplexer` wrapping that instance, forwarding all keyword arguments to the `TubeMultiplexer` constructor.