feat(tvix/nix-compat/wire): introduce BytesWriter

This deals with writing byte packets of larger sizes to an underlying
AsyncWrite.

Its constructor receives the expected size. It also deals with writing
padding if flush/shutdown is called after writing all the payload.

Change-Id: I8acbf992467f3862ffb8c7d669e8c0c8eced14c1
Reviewed-on: https://cl.tvl.fyi/c/depot/+/11355
Reviewed-by: picnoir picnoir <picnoir@alternativebit.fr>
Autosubmit: flokli <flokli@flokli.de>
Reviewed-by: Brian Olsen <me@griff.name>
Tested-by: BuildkiteCI

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

@@ -4,6 +4,11 @@
 #[cfg(feature = "async")]
 pub mod bytes;
 
+#[cfg(feature = "async")]
+mod bytes_writer;
+#[cfg(feature = "async")]
+pub use bytes_writer::BytesWriter;
+
 #[cfg(feature = "async")]
 pub mod primitive;