nova: Remove `Ec::cont` to make thread switching easier to understand

TODO: Reference solution for 12threads in the private repo will also need to be changed.

Originally, Ec::cont was (afaict) designed to allow switching between threads in different processes (with different virtual memory space). For switching between threads inside a single process, it is not needed, but it must be understood to correctly use Ec::make_current, which seems (rightfully) confusing to many students.

This commit also gets rid of the indirect call of Ec::root_invoke, so now hopefully it should be easier to understand the program flow during boot.