auto merge of #18756 : jbcrail/rust/add-enum-set-bitxor, r=alexcrichton

I implemented BitXor, and also added tests for BitAnd and BitXor.

cc #18424

diff --cc src/libcollections/enum_set.rs
index b13e45099ee29c629257771055632fbeeae742f0,b2b26914b441a6f449ca2a1a733e659a9983d052..ee0c4644b36a59f28bee7dea5dbd5797fa434fb0
--- 1/src/libcollections/enum_set.rs
--- 2/src/libcollections/enum_set.rs
+++ b/src/libcollections/enum_set.rs
@@@ -16,8 -16,8 +16,7 @@@
  use core::prelude::*;
  use core::fmt;
  
- // FIXME(conventions): implement BitXor
  // FIXME(contentions): implement union family of methods? (general design may be wrong here)
 -// FIXME(conventions): implement len
  
  #[deriving(Clone, PartialEq, Eq, PartialOrd, Ord, Hash)]
  /// A specialized `Set` implementation to use enum types.