Split monolithic raw crates into sys crates

Creating a crate for bwd-gc highlights the fact that it would be nice to
fix 2!

The file blocklist is a lost less unmaintainable then the more
fine-grained one we had before.

Fix #9

nix-bindings-expr-sys/build.rs

@@ -0,0 +1,42 @@
+use std::path::PathBuf;
+
+#[derive(Debug)]
+struct StripNixPrefix;
+
+impl bindgen::callbacks::ParseCallbacks for StripNixPrefix {
+    fn item_name(&self, name: &str) -> Option<String> {
+        name.strip_prefix("nix_").map(String::from)
+    }
+}
+
+fn main() {
+    println!("cargo:rerun-if-changed=include/nix-c-expr.h");
+    println!("cargo:rustc-link-lib=nixexprc");
+
+    let mut args = Vec::new();
+    for path in pkg_config::probe_library("nix-expr-c")
+        .unwrap()
+        .include_paths
+        .iter()
+    {
+        args.push(format!("-I{}", path.to_str().unwrap()));
+    }
+
+    let out_path = PathBuf::from(std::env::var("OUT_DIR").unwrap());
+
+    let bindings = bindgen::Builder::default()
+        .header("include/nix-c-expr.h")
+        .clang_args(args)
+        .parse_callbacks(Box::new(bindgen::CargoCallbacks::new()))
+        .parse_callbacks(Box::new(StripNixPrefix))
+        // Blocklist symbols from nix-bindings-util-sys
+        .blocklist_file(".*nix_api_util\\.h")
+        // Blocklist symbols from nix-bindings-store-sys
+        .blocklist_file(".*nix_api_store\\.h")
+        .generate()
+        .expect("Unable to generate bindings");
+
+    bindings
+        .write_to_file(out_path.join("bindings.rs"))
+        .expect("Couldn't write bindings!");
+}