refactor(tvix/derivation): minor structure simplification

Fixes some module comments and embeds the `compress_hash` function in
the `derivation` module, as it was not used outside of this module
anyways.

Change-Id: I6c5c92b3f0c03c2cdcbcfc2f813909a968c4d44c
Reviewed-on: https://cl.tvl.fyi/c/depot/+/7905
Tested-by: BuildkiteCI
Reviewed-by: flokli <flokli@flokli.de>

tvix/derivation/src/write.rs

@@ -1,3 +1,8 @@
+//! This module implements the serialisation of derivations into the
+//! [ATerm][] format used by C++ Nix.
+//!
+//! [ATerm]: http://program-transformation.org/Tools/ATermFormat.html
+
 use crate::output::Output;
 use crate::string_escape::escape_string;
 use std::collections::BTreeSet;