Root projection

[CeleritasCelery]

This really needs to be proc macro to auto derive since implementing is unsafe, but that is not yet implemented.

We get a RootRef by using RootOwner on Root. Root projection let's take a RootRef of a struct and get RootRef of the fields. This is similar to pin-project with a few big differences. First is that a Pin<P> hold a pointer to some data, but our type RootRef<T> hold the data itself. This means we can never return an owned RootRef (like you can we Pin) but instead return only references from our projections.

For example here is what is implemented for hashmap

https://github.com/CeleritasCelery/rune/blob/7136b74386a4586537ffa59c3be4849cb76354fe/src/arena/root.rs#L357-L384

We get the interior value which does not have RootRef on it (similar to how we would get a &mut T from a Pin<P> with get_mut_unchecked. Then we call the function we need (like get) and cast the return value &V directly to &RootRef<V>. This let's us use Root-only methods on V because we have transitively rooted it.

Why this is sound

RootRef is repr(transparent) so it is safe to cast it a wrapper, since we know the parent is rooted. The "child" reference borrows from the "parent", so it will follow rusts normal borrowing rules. This is the same logic that std::cell::Cell's as_slice_of_cells uses.