Rewrite rust_start in Rust

[brson]

Upcalls are slowly being converted to rust. We'll want to convert rust_start (the runtime entry point) as well, and I think we're almost to the point where we can do this. Some difficulties:

• It will run on a big native stack, so the compiler will have to set up the stack limit before main
• Any call that requires the task pointer will fail hard. In particular this means we can't do any managed allocations

We can start by just immediately calling into the existing C++ rust_start function.

[brson]

Let's do this with a lang item #[lang = "start"].

[brson]

Done.