Rename nix-bindings-bindgen-raw to nix-bindings-util-sys

Adopt idiomatic Rust `-sys` crate naming convention, aligning with
the direction in PR #37 without adopting the full crate splitting.

nix-bindings-expr/src/value/__private.rs

@@ -1,6 +1,6 @@
 //! Functions that are relevant for other bindings modules, but normally not end users.
 use super::Value;
-use nix_bindings_bindgen_raw as raw;
+use nix_bindings_util_sys as raw;
 
 /// Take ownership of a new [`Value`].
 ///