feat(core): remove upgrade::transfer module

As described in #4011, these utility functions don't belong in `libp2p-core`. Users can use `quick-protobuf-codec` if they need to write varint-prefixed protobuf messages. For writing varint-prefixed bytes, the `unsigned-varint` crate offers a various codec implementations.

`libp2p-core` is the base dependency of all other crates. Thus, we should only expose items there that are actually needed by all other crates. For implementation details like how bytes are written, downstream crates (including users) should reach for other crates.

Depends-On: #4787.
Resolves: #4011.

Pull-Request: #4788.

core/CHANGELOG.md

@@ -4,6 +4,9 @@
   See [PR 4695](https://github.com/libp2p/rust-libp2p/pull/4695).
 - Remove deprecated functions from `ListenerId`.
   See [PR 4736](https://github.com/libp2p/rust-libp2p/pull/4736).
+- Remove `upgrade::transfer` module.
+  See [issue 4011](https://github.com/libp2p/rust-libp2p/issues/4011) for details.
+  See [PR 4788](https://github.com/libp2p/rust-libp2p/pull/4788).
 
 ## 0.40.1
 

core/src/upgrade.rs

@@ -64,7 +64,6 @@ mod error;
 mod pending;
 mod ready;
 mod select;
-mod transfer;
 
 pub(crate) use apply::{
     apply, apply_inbound, apply_outbound, InboundUpgradeApply, OutboundUpgradeApply,
@@ -73,11 +72,7 @@ pub(crate) use error::UpgradeError;
 use futures::future::Future;
 
 pub use self::{
-    denied::DeniedUpgrade,
-    pending::PendingUpgrade,
-    ready::ReadyUpgrade,
-    select::SelectUpgrade,
-    transfer::{read_length_prefixed, read_varint, write_length_prefixed, write_varint},
+    denied::DeniedUpgrade, pending::PendingUpgrade, ready::ReadyUpgrade, select::SelectUpgrade,
 };
 pub use crate::Negotiated;
 pub use multistream_select::{NegotiatedComplete, NegotiationError, ProtocolError, Version};