net: remove packet pool.

The pool was prone to deadlocks, especially due to having a single pool
for rx+tx. If the pool got full with rx'd packets it would deadlock because
processing a rx packet requires doing another allocation on the pool, for
the possibly tx'd response, before deallocating the rx'd packet.

This also allows Device impls to allocate the packet memory in a particular
RAM kind, if needed for example to do DMA.

The `Device` trait is now token-based, like smoltcp's. In the end, this
is better because it allows callers to manage memory however they want (including
with a pool if they want to).

embassy-net/src/tcp.rs

@@ -9,8 +9,8 @@ use smoltcp::time::Duration;
 use smoltcp::wire::{IpEndpoint, IpListenEndpoint};
 
 use super::stack::Stack;
+use crate::device::Device;
 use crate::stack::SocketStack;
-use crate::Device;
 
 #[derive(PartialEq, Eq, Clone, Copy, Debug)]
 #[cfg_attr(feature = "defmt", derive(defmt::Format))]