upack doesn't roundtrip `Ext` nested inside another `Msg`

Here's a minimal roundtripping failure:

```scala
val msg = upack.Arr(upack.Ext(33, new Array[Byte](4)), upack.False)
val bytes = upack.writeToByteArray(msg)
val msg2 = upack.read(bytes)
```

This decodes as `upack.Arr(upack.Ext(33, new Array[Byte](4)), upack.Int32(33))` (the last bit is the problem: `upack.Int32(33)` instead of `upack.False`).