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-store/src/path.rs

@@ -1,11 +1,11 @@
 use std::ptr::NonNull;
 
 use anyhow::Result;
-use nix_bindings_bindgen_raw as raw;
 use nix_bindings_util::{
     result_string_init,
     string_return::{callback_get_result_string, callback_get_result_string_data},
 };
+use nix_bindings_util_sys as raw;
 
 pub struct StorePath {
     raw: NonNull<raw::StorePath>,