security(rust): forbid non-ascii identifiers (#3615)

Rust supports non-ascii identifiers, but we don't use them in Zebra.

But similar non-ascii characters can be a security risk,
particularly when using some editors or GitHub reviews.

.cargo/config.toml

@@ -5,6 +5,7 @@ rustflags = [
 
     # High-risk code
     "-Dunsafe_code",
+    "-Dnon_ascii_idents",
 
     # Potential bugs
     #