refact: Make the callback convert to Result<String> immediately

This is slightly easier to use than the previous pattern that was
always followed up by the same conversions.

(cherry picked from commit 756c080730cd4fa81d4c0e3a99688cbe8debe57f)

rust/nix-util/src/lib.rs

@@ -1,3 +1,4 @@
 pub mod context;
 pub mod settings;
+#[macro_use]
 pub mod string_return;