Design rv6 user domain entry point interface

Created by: tjhu

Design a clean and unified way for the rv6 userspace domains to define their entry points. For example, we can have something like fn main(s: RRef<Xv6SysCalls>, args: RRef<Vec<String>>). Question: how are we going to enforce this interface so that the domains are not able to use any arbitrary function as the entry point?

• Define an interface
• Enforce the interface