script: Rewrite the layout DOM wrappers (#44114)

This change reworks the layout DOM wrappers so that they are simpler and
easier to reason about. The main changes here:

**Combine layout wrappers into one interface:**

 - `LayoutNode`/`ThreadSafeLayoutNode` is combined into `LayoutNode`:
   The idea here is that `LayoutNode` is always thread-safe when used in
   layout as long as no `unsafe` calls are used. These interfaces
   only expose what is necessary for layout.
 - `LayoutElement`/`ThreadSafeLayoutElement` is combined into
   `LayoutElement`: See above.

**Expose two new interfaces to be used *only* with `stylo` and
`selectors`:**

`DangerousStyleNode` and `DangerousStyleElement`. `stylo`
and `selectors` have a different way of ensuring safety that is
incompatible with Servo's layout (access all of the DOM tree anywhere,
but ensure that writing only happens from a single-thread). These types
only implement things like `TElement`, `TNode` and are not intended to
be used by layout at all.

All traits and implementations are moved to files that are named after
the struct or trait inside them, in order to better understand what one
is looking at.

The main goals here are:

 - Make it easier to reason about the safe use of the DOM APIs.
 - Remove the interdependencies between the `stylo` and `selectors`
   interface implementations and the layout interface. This helps
   with the first point as well and makes it simpler to know where
   a method is implemented.
 - Reduce the amount of code.
 - Make it possible to eliminate `TrustedNodeAddress` in the future.
 - Document and bring the method naming up to modern Rust conventions.

This is a lot of code changes, but is very well tested by the WPT tests.
Unfortunately, it is difficult to make a change like this iteratively.
In addition, this new design comes with new documentation at
servo/book#225.

Testing: This should not change behavior so should be covered by
existing
WPT tests.

Signed-off-by: Martin Robinson <mrobinson@fastmail.fm>

components/script/dom/html/htmltextareaelement.rs

@@ -11,7 +11,7 @@ use fonts::{ByteIndex, TextByteRange};
 use html5ever::{LocalName, Prefix, local_name, ns};
 use js::context::JSContext;
 use js::rust::HandleObject;
-use layout_api::wrapper_traits::{ScriptSelection, SharedSelection};
+use layout_api::{ScriptSelection, SharedSelection};
 use servo_base::text::Utf16CodeUnitLength;
 use style::attr::AttrValue;
 use stylo_dom::ElementState;