Use string primitive to avoid a heap allocation

src/main.rs

@@ -21,7 +21,7 @@ fn main() {
     // Disallowed_chars is an optional argument
     let disallowed_chars: Option<&String> = args.get(2);
 
-    let allowed_chars: String = String::from("abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ1234567890-=`~!@#$%^&*()_+[]\\{}|;':,./<>?");
+    let allowed_chars: &str = "abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ1234567890-=`~!@#$%^&*()_+[]\\{}|;':,./<>?";
 
     let mut allowed_chars_set: HashSet<char> = allowed_chars.chars().collect();