Replace unsafe raw pointers to the node tree

[matthunz]

• Fix size constraint for https://github.com/servo/stylo/blob/789ab89c30d94e25f514c19c079a00a6ab583638/style/sharing/mod.rs#L612

The methods for Node currently seem unsafe, as a Node can outlive its tree field of *mut Slab<Node>. It is also possible to obtain mutable references to the same node.

• Replaces the current unsafe method of storing raw pointers to the tree of each node:

  struct Node {
    tree: *mut Slab<Node>,
    ...
  }

  With a container handle struct safely immutably borrowing from both:

  struct Node { ... }
  
  struct Handle<'a> {
      node: &'a Node,
      tree: &'a Slab<Node>,
  }

• Adds documentation for Node fields and methods
• Refactors and removes some dead code

[nicoburns]

@matthunz This mostly all looks pretty good, but:

• Can we split it into separate PRs: in particular, the documentation and zoom changes seem standalone and uncontroversial, so would be good to get those merged!
• Can we call the type NodeHandle?

You might also want to consider making the type:

struct NodeHandle<'a> {
    doc: &'a Document,
    node: &'a Node,
}

Which would allow the type to implement more things. I think we should also look at adding a

struct NodeHandleMut<'a> {
    doc: &mut 'a Document,
    node_id: NodeId,
}

which would allow a lot of the methods from Document to moved to NodeHandleMut.

[matthunz]

Thanks for the review!

• Can we split it into separate PRs: in particular, the documentation and zoom changes seem standalone and uncontroversial, so would be good to get those merged!

Definitely! I think I got sucked into a bit of a rabbit hole here... I'll try to factor out the less controversial stuff

• Can we call the type NodeHandle?

Ooo yeah I actually like that a lot better

You might also want to consider making the type:

struct NodeHandle<'a> {
    doc: &'a Document,
    node: &'a Node,
}

That sounds interesting too, it does sound like we'd have more flexibility to add things. I also really like the mutable handle for the same reasons.

Just to clarify on that todo task though unfortunately it looks like stylo only excepts pointer-sized element types (specifically usize-sized, which sounds like a provenance issue so maybe we could submit a PR there 🤔). I was thinking we could do impl TElement for &'a NodeHandle<'a> but that's tricky with Traverser... I think it's possible with stylo as-is but I'm a little stuck so far