Optimize unreachable branches

[zslayton]

Use something like this crate to help the compiler optimize away bits like:

   fn as_ref(&self) -> &T {
    match self.value.as_ref() {
      Some(v) => v,
      // This is unreachable but is currently preserved by LLVM
      None => panic!("Recycled<T> smartpointer missing its value.")
    }
  }

[zslayton]

At present, it is possible for the Recycled and RcRecycled types to be missing their wrapped value by the time they reach the destructor because of the detach method, which unwraps it. Because of that, it is only possible to add this optimization in the as_ref and as_mut methods. Investigating whether this can be revised.