Requires a complete change of the HTML tree, to have interior mutability
and be able to return borrowed data from a Handle.
It is now a Rc-based tree where each node has a weak reference to
its parent and strong references to its children.
This results ina few breaking changes to the public API.
- Don't instruct docs.rs to set it, it does so by default now:
https://github.com/rust-lang/docs.rs/pull/2390
- Because of that, check-cfg also recognizes it and does not need
extra configuration either.
