feat(nix-compat/nar/reader): async support

This is a first cut at the async NAR reader, with some rough edges.
Poisoning is left unimplemented for now, pending future work.

Change-Id: Ifaafe0581a5e0e165a13357b909fb441f7bd8bab
Reviewed-on: https://cl.tvl.fyi/c/depot/+/11524
Reviewed-by: flokli <flokli@flokli.de>
Tested-by: BuildkiteCI

tvix/nix-compat/src/wire/bytes/mod.rs

@@ -33,7 +33,7 @@ const LEN_SIZE: usize = 8;
 ///
 /// This buffers the entire payload into memory,
 /// a streaming version is available at [crate::wire::bytes::BytesReader].
-pub async fn read_bytes<R>(
+pub async fn read_bytes<R: ?Sized>(
     r: &mut R,
     allowed_size: RangeInclusive<usize>,
 ) -> std::io::Result<Vec<u8>>