refactor(tvix/nix-compat): reorganize wire and bytes

Move everything bytes-related into its own module, and re-export
both bytes and primitive in a flat space from wire/mod.rs.

Expose this if a `wire` feature flag is set. We only have `async` stuff
in here.

Change-Id: Ia4ce4791f13a5759901cc9d6ce6bd6bbcca587c7
Reviewed-on: https://cl.tvl.fyi/c/depot/+/11389
Autosubmit: flokli <flokli@flokli.de>
Reviewed-by: raitobezarius <tvl@lahfa.xyz>
Tested-by: BuildkiteCI
Reviewed-by: Brian Olsen <me@griff.name>

tvix/nix-compat/Cargo.toml

@@ -4,7 +4,10 @@ version = "0.1.0"
 edition = "2021"
 
 [features]
-async = ["futures-util", "tokio", "pin-project-lite"]
+# async NAR writer
+async = ["futures-util"]
+# code emitting low-level packets used in the daemon protocol.
+wire = ["tokio", "pin-project-lite"]
 
 [dependencies]
 bitflags = "2.4.1"