Rename channel to mpmc

I've renamed the channel module for the MPMC as mpmc. There was a previous debate about this, but I feel that the strategy here avoids importing `channel::channel`. The change leaves `signal::Signal`, but I think that's ok. It is all a bit subjective of course. The bottom line for me is that I really like the term mpmc - it means something to me and aligns with broader naming e.g. in Tokio.

examples/stm32l5/src/bin/usb_ethernet.rs

@@ -8,7 +8,7 @@ use core::task::Waker;
 use defmt::*;
 use defmt_rtt as _; // global logger
 use embassy::blocking_mutex::raw::ThreadModeRawMutex;
-use embassy::channel::Channel;
+use embassy::channel::mpmc::Channel;
 use embassy::executor::Spawner;
 use embassy::util::Forever;
 use embassy_net::tcp::TcpSocket;