All the FFI functions have fancy types now and use references instead
of raw pointers for arguments.

Now, a CFRef<T> value represents a refcounted value to be released on Drop,
CFShared<T> represents a shared value that can be retained to produce a new
CFRef<T> (this will be a useful distinction for mutable types, where a
bare T shouldn't be retainable to keep unique ownership).

To simplify things and avoid either having wrappers of wrappers of all
methods behind traits, I put the safe and thin methods directly on the
opaque FFI types.

Let's begin the infinite bikeshedding!

This type provides DerefMut access to the underlying Core Foundation type.
Will be used for CFMutableString and friends.