Namely:
* Added conversion traits both to and from the various vector types,
analogous to how `str::MaybeOwned` works with `str::IntoMaybeOwned`
and `str::Str`. This led me to add the `FixedLen` variant of
`MaybeOwnedVector` for interoperability with `std::slice`.
* Revised client example code to make use of `into_maybe_owned`
* Added implementations of `Show` and `CloneableVector` for
`MaybeOwnedVector`.
* As suggested by kballard, added `into_vec` method that is analogous
to `CloneableVector::into_owned` except it produces a `Vec` rather
than a `~[T]`.
