Drop dep tokio's io-util feat as it broke MSRV and isn't useful

[TheBlueMatt]

We use tokio's io-util feature to provide the
Async{Read,Write}Ext traits, which allow us to simply launch a read future or poll_write directly as well as split the TcpStream into a read/write half. However, these traits aren't actually doing much for us - they are really just wrapping the readable future (which we can trivially use ourselves) and poll_write isn't doing anything for us that poll_write_ready can't.

Similarly, the split logic is actually just Arcing the TcpStream and busy-waiting when an operation is busy to prevent concurrent reads/writes. However, there's no reason to prevent concurrent access at the stream level - we aren't ever concurrently writing or reading (though we may concurrently read and write, which is fine).

Worse, the io-util feature broke MSRV (though they're likely to fix this upstream) and carries two additional dependencies (only one on the latest upstream tokio).

Thus, we simply drop the dependency here.

Fixes #2527.

[TheBlueMatt]

Oops, also had to test-only pin memchr anyway cause regex uses it, but that's okay, this PR still removes the dependency on bytes, and memchr on older tokio in non-dev environments.

[jkczyz]

CI still unhappy 😭

+ cargo check --verbose --color always
    Updating crates.io index
    Updating git repository `[https://github.com/arik-so/rust-musig2`](https://github.com/arik-so/rust-musig2%60)
 Downloading crates ...
  Downloaded memchr v2.6.0
error: failed to parse manifest at `/home/runner/.cargo/registry/src/github.com-1ecc6299db9ec823/memchr-2.6.0/Cargo.toml`

Caused by:
  failed to parse the `edition` key

Caused by:
  this version of Cargo is older than the `2021` edition, and only supports `2015` and `2018` editions.
Error: Process completed with exit code 101.

[TheBlueMatt]

Grr rust-bitcoin/rust-bitcoin#2034

[TheBlueMatt]

In any case, IMO we should still land this while we work through core2.

[wpaulino]

Feel free to squash, LGTM

[jkczyz]

LGTM

[TheBlueMatt]

Squashed with the one extra space removed:

$ git diff-tree -U1 97b6c6f6 afc5a02f
diff --git a/lightning-net-tokio/src/lib.rs b/lightning-net-tokio/src/lib.rs
index 55906c64f..6e2ea3f14 100644
--- a/lightning-net-tokio/src/lib.rs
+++ b/lightning-net-tokio/src/lib.rs
@@ -467,3 +467,3 @@ impl peer_handler::SocketDescriptor for SocketDescriptor {
 		// there's room in the kernel buffer, or otherwise create a new Waker with a
-		// SocketDescriptor  in it which can wake up the write_avail Sender, waking up the
+		// SocketDescriptor in it which can wake up the write_avail Sender, waking up the
 		// processing future which will call write_buffer_space_avail and we'll end up back here.
$