Don't copy a 200,000 element vec

src/lib.rs

             tld.map(|tld| {
                 let word = self.get_random_word(&tld);
                 if word.is_some() {
-                    let mut word = word.unwrap();
+                    let mut word = word.unwrap().to_string();
                     word.truncate(word.len() - 2);
                     domain = format!("{}.{}", word, tld);
                 }
             .map(|v| String::from(v))
     }
 
-    fn get_random_word(&self, tld: &str) -> Option<String> {
+    fn filter_words(&self, tld: &str) -> Vec<&str> {
         self.words
-            .clone()
-            .into_iter()
+            .iter()
             .filter(|word| word.ends_with(tld))
-            .collect::<Vec<String>>()
+            .map(|i| &i[..])
+            .collect()
+    }
+
+    fn get_random_word(&self, tld: &str) -> Option<&str> {
+        self.filter_words(tld)
             .choose(&mut rand::thread_rng())
-            .map(|v| v.to_string().to_lowercase())
+            .map(|w| *w)
     }
 }