Add cppgc bindings

[littledivy]

https://v8.dev/blog/high-performance-cpp-gc

Oilpan in Deno design doc: https://www.notion.so/denolandinc/Oilpan-cppgc-in-Deno-e194f4268e9f4135ba97610ff7d3a949?pvs=4

Oilpan can be used to implement GC'able resources in Deno

Closes #933

[lucacasonato]

Will need a layer ontop in deno_core to make more rust idiomatic I think. Looking good

[mmastrac]

First of all, this is awesome!

I wonder if we should keep this interface low-level for now. There's a few Gc container types in Rust already and we may want to experiment with different designs until we find something that's ergonomic for what we need.

https://manishearth.github.io/blog/2021/04/05/a-tour-of-safe-tracing-gc-designs-in-rust/

I think the ideal level of interface here might be the ability to create a Boxed T with the correct C++ vtable that offers the low-level tracing API (unopinionated at this level), and automatically drops its inner object when the GC finds it goes out of scope.

What we'd then do is build a library (maybe in the deno_core project?) to offer a Gc<T> type based on the low-level interface.

[aapoalas]

nitpick: &mut *platform as *mut Platform perhaps? Or does SharedRef possibly have an as_ptr or as_mut_ptr API?

[littledivy]

SharedRef does not deref as mutable. This is a common pattern for passing pointers where Rust owernship does not matter:

rusty_v8/src/V8.rs

Line 188 in 12dca0c

v8__V8__InitializePlatform(&*platform as *const Platform as *mut _)

[mmastrac]

Unrelated to this PR, but we should probably have as_ptr and as_mut_ptr convenience methods on SharedRef to avoid the need to deref/ref and cast to pointer.

[mmastrac]

LGTM!