This patch moves the data members of URL to an internal URL::Data struct
that is also reference-counted. URL then uses a CopyOnWrite<T> template
to give itself copy-on-write behavior.
This means that URL itself is now 8 bytes per instance, and copying is
cheap as long as you don't mutate.
This shrinks many data structures over in LibWeb land. As an example,
CSS::ComputedValues goes from 3024 bytes to 2288 bytes per instance.
This URL library ends up being a relatively fundamental base library of
the system, as LibCore depends on LibURL.
This change has two main benefits:
* Moving AK back more towards being an agnostic library that can
be used between the kernel and userspace. URL has never really fit
that description - and is not used in the kernel.
* URL _should_ depend on LibUnicode, as it needs punnycode support.
However, it's not really possible to do this inside of AK as it can't
depend on any external library. This change brings us a little closer
to being able to do that, but unfortunately we aren't there quite
yet, as the code generators depend on LibCore.
