Use a non-dropping enum directly

Don't use NoDrop as a separate abstraction: Then we have two drop flags,
one for ArrayVec and one for NoDrop. Instead import the logic from
NoDrop. The result is a much smaller ArrayVec value.

tests/tests.rs

@@ -127,15 +127,15 @@ fn test_is_send_sync() {
 #[test]
 fn test_compact_size() {
     // Future rust will kill these drop flags!
-    // 4 elements size + 1 len + 1 enum tag + [1 drop flag] + [1 drop flag nodrop]
+    // 4 elements size + 1 len + 1 enum tag + [1 drop flag]
     type ByteArray = ArrayVec<[u8; 4]>;
     println!("{}", mem::size_of::<ByteArray>());
-    assert!(mem::size_of::<ByteArray>() <= 8);
+    assert!(mem::size_of::<ByteArray>() <= 7);
 
-    // 12 element size + 1 len + 1 drop flag + 2 padding + 1 enum tag + 3 padding
+    // 12 element size + 1 enum tag + 3 padding + 1 len + 1 drop flag + 2 padding
     type QuadArray = ArrayVec<[u32; 3]>;
     println!("{}", mem::size_of::<QuadArray>());
-    assert!(mem::size_of::<QuadArray>() <= 24);
+    assert!(mem::size_of::<QuadArray>() <= 20);
 }
 
 #[test]