Test the deque a bit. Give it a get-by-index method. Fix two uncovered state-calculation bugs --- one decently, the other with an ugly hack. Bug on the latter coming right up.

src/test/run-pass/lib-deque.rs

@@ -0,0 +1,17 @@
+// -*- rust -*-
+
+use std;
+import std.deque;
+
+fn main() {
+  let deque.t[int] d1 = deque.create[int]();
+  check (d1.size() == 0u);
+  d1.add_front(17);
+  d1.add_front(42);
+  d1.add_back(137);
+  check (d1.size() == 3u);
+  d1.add_back(137);
+  check (d1.size() == 4u);
+  /* FIXME (issue #133):  We should check that the numbers come back
+   * to us correctly once the deque stops zeroing them out. */
+}