Audit safety of the Rust bindings

[nox]

Some issues I found:

• Check whether some Context methods should take a &mut self receiver;
• check whether Context::unref should be a Drop implementation;
• clarify what happens to the unregistered callback in Context::clear_state_callback;
• Context::set_state_callback should be made to not need a *mut c_void parameter;
• Context::connect should be unsafe (can pass 0xdeadbeef as *const _ for its api argument);
• Context::drain should be made to not need a *mut c_void parameter;
• Context::rttime_new should be made to not need a *mut c_void parameter;
• Context::get_server_info should be made to not need a *mut c_void parameter;
• Context::get_sink_info_by_name should be made to not need a *mut c_void parameter;
• Context::get_sink_info_list should be made to not need a *mut c_void parameter;
• Context::get_sink_input_info should be made to not need a *mut c_void parameter;
• Context::get_source_info_list should be made to not need a *mut c_void parameter;
• Context::set_seek_input_volume should be made to not need a *mut c_void parameter;
• Context::subscribe should be made to not need a *mut c_void parameter;
• clarify what happens to the unregistered callback in Context::clear_subscribe_callback;
• Context::set_subscribe_callback should be made to not need a *mut c_void parameter;
• check whether some MainloopApi methods should take a &mut self receiver;
• MainloopApi::once should be made to not need a *mut c_void parameter;
• MainloopApi::time_free should be unsafe (can pass 0xdeadbeef as *mut _ for its e argument);
• check whether some Stream methods should take a &mut self receiver;
• check whether Stream::unref should be a Drop implementation;
• Stream::begin_write should probably return a safe wrapper instead of a raw pointer;
• clarify whether Stream::begin_write can be called again before completion of last write;
• Stream::write should be unsafe (can pass 0xdeadbeef as *mut _ for its data argument);
• clarify what Stream::drop is an intrinsic method and not an actual Drop implementation;
• Stream::update_timing_info should be made to not need a *mut c_void parameter;
• clarify what happens to the unregistered callback in Stream::clear_state_callback;
• Stream::set_state_callback should be made to not need a *mut c_void parameter;
• clarify what happens to the unregistered callback in Stream::clear_write_callback;
• Stream::set_write_callback should be made to not need a *mut c_void parameter;
• clarify why happens to the unregistered callback in Stream::clear_read_callback;
• Stream::set_read_callback should be made to not need a *mut c_void parameter;
• Stream::cork should be made to not need a *mut c_void parameter;
• check whether some ThreadedMainloop methods should take a &mut self receiver;
• clarify why ThreadedMainloop::is_null is needed (AFAICT the pointer should not be null ever).

More stuff will probably surface once we start handling these.