Add a `graphviz` crate for making .dot files to layout and render graphs.
Updated maybe_owned_vec with review feedback.

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]`.
Placate rustdocs testable-by-default code blocks.
auto merge of #13749 : pnkfelix/rust/add-libgraphviz-crate, r=alexcrichton

Add a `graphviz` crate for making .dot files to layout and render graphs.

(This is a precursor to other work to render control-flow graphs from within rustc itself; but this crate should be independently usable, since it abstracts over the client's graph-representation and labeling method.)