Traits other than NoCell permit UnsafeCells

TODO:
- Require `T: NoCell` and `U: NoCell` in `transmute_ref!` and
  `transmute_mut!`
- Tests
  - Exercise transmuting `UnsafeCell`s by value
- Time to rename `AsBytes` to `ToBytes`?

Closes #251

src/byteorder.rs

@@ -655,7 +655,7 @@ mod tests {
     use compatibility::*;
 
     // A native integer type (u16, i32, etc).
-    trait Native: Arbitrary + FromBytes + AsBytes + Copy + PartialEq + Debug {
+    trait Native: Arbitrary + FromBytes + AsBytes + NoCell + Copy + PartialEq + Debug {
         const ZERO: Self;
         const MAX_VALUE: Self;
 
@@ -692,7 +692,7 @@ mod tests {
     }
 
     trait ByteArray:
-        FromBytes + AsBytes + Copy + AsRef<[u8]> + AsMut<[u8]> + Debug + Default + Eq
+        FromBytes + AsBytes + NoCell + Copy + AsRef<[u8]> + AsMut<[u8]> + Debug + Default + Eq
     {
         /// Invert the order of the bytes in the array.
         fn invert(self) -> Self;