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/nrf/src/bin/usb_hid_keyboard.rs

@@ -6,7 +6,7 @@
 use core::mem;
 use core::sync::atomic::{AtomicBool, Ordering};
 use defmt::*;
-use embassy::channel::Signal;
+use embassy::channel::signal::Signal;
 use embassy::executor::Spawner;
 use embassy::interrupt::InterruptExt;
 use embassy::time::Duration;