refactor(tvix/nix-compat): move nar writer to tokio

There's little reason to keep the nar writer using Async{Read,Write}
traits from futures, while everything else async in tvix (and
nix-compat) uses tokio.

Change-Id: I8cd1efcd0dd5bb76471de997603c7b701a5095de
Reviewed-on: https://cl.tvl.fyi/c/depot/+/11391
Tested-by: BuildkiteCI
Reviewed-by: raitobezarius <tvl@lahfa.xyz>
Reviewed-by: Brian Olsen <me@griff.name>

tvix/nix-compat/Cargo.toml

@@ -5,7 +5,7 @@ edition = "2021"
 
 [features]
 # async NAR writer
-async = ["futures-util"]
+async = ["tokio"]
 # code emitting low-level packets used in the daemon protocol.
 wire = ["tokio", "pin-project-lite"]
 
@@ -16,7 +16,6 @@ data-encoding = "2.3.3"
 ed25519 = "2.2.3"
 ed25519-dalek = "2.1.0"
 enum-primitive-derive = "0.3.0"
-futures-util = { version = "0.3.30", features = ["io"], optional = true }
 glob = "0.3.0"
 nom = "7.1.3"
 num-traits = "0.2.18"