New architecture

[liquidev]

aka. decoupling stuff into smaller parts.

The plan is to split the monolithic Engine into many smaller components:

• There'll be a new Engine that only contains an Environment. It will be used for compiling Scripts.
  • Scripts will no longer hold a mutable reference to the engine, and instead will use a unique ID assigned to the engine at its creation to ensure correctness.
• Engines will also be used for creating Isolates, which will be the actual execution environments.
  • An Isolate is like a reusable replacement for a Fiber, except as the name suggests - it's completely isolated from other isolates. Each isolate has its own GC and global storage.
    • Scripts will no longer be able to write global variables, only read from them. Global variable shadowing will also not be allowed.
      • I'm also considering making global variables be part of the Engine, because hey they're immutable anyways. But we'll see how that goes.
• In addition there will also be a Registry type - a dynamically typed storage container, basically a HashMap<TypeId, RefCell<Box<dyn Any>>>.
  • There will be two registries: the static registry (part of the Engine,) and the isolate registry (one per Isolate). Foreign functions will have a way of accessing these registries and mutating what's inside them.
  • This mechanism will be modeled to fit the requirements for implementing require.