Appending

- Adds `+` and `+=` over (`ExtensibleCollectionType`, `ReducibleType`).
- Drops `Stream`’s `SequenceType` conformance, which was creating needless ambiguity. Note that this means that to use `Stream` with `for`…`in`, you will now have to wrap it in a call to `sequence`:

  ```swift
  for each in sequence(stream) {
    …
  }
  ```

- Fixes #28.