Remove macro_use

As of Rust 2018, macro_use is no longer required in most circumstances.
I think it is generally a good idea to remove these when not needed, to
stop them from polluting the crate's global namespace.
https://doc.rust-lang.org/edition-guide/rust-2018/macros/macro-changes.html#macro_rules-style-macros

nix-rust/src/store/path.rs

@@ -138,6 +138,7 @@ impl fmt::Display for StorePathName {
 #[cfg(test)]
 mod tests {
     use super::*;
+    use assert_matches::assert_matches;
 
     #[test]
     fn test_parse() {