pub fn init(closure: Closure) -> Fiber {
        let mut fiber = Fiber {
            closure,
            stack: Stack::init(),
            ip: 0,
        };
        fiber.stack.declare(fiber.closure.lambda.decls);
        return fiber;
    }
Initialize a new Fiber.
To run the Fiber, a lambda must be passed to it through `run`.
passerine/src/vm/fiber.rs