title: List with fast contains
body: <p>I wonder if there's a <code>List</code> implementation allowing fast <code>contains</code>. I'm working with quite a long <code>List</code> and I can't switch to <code>Set</code> since I need the access by index. I can ignore the performance, which may be acceptable now and may or may not be acceptable in the future. I can create a <code>HashSet</code> and do all modifying operations on both, but doing it manually is quite boring and error prone.</p>
  
  <p>I know that it's impossible to have a class working like both <code>List</code> and <code>Set</code> (because of the different <code>equals</code> semantics), but I wonder if there's <code>List</code> implementing <code>RandomAccess</code> and employing an <code>HashSet</code> for speeding up <code>contains</code>.</p>